Fix Kotlin and Java interop issues. (#343)

The following PR fixes interop issues between Kotlin and Java.

Currently, there is no way to go from `Command.Builder` -> `MutableCommandBuilder`, nor from `MutableCommandBuilder` -> `Command.Builder`.

This PR fixes it by introducing
- A new constructor for `MutableCommandBuilder`:
  
  ```kotlin
  public constructor(commandBuilder: Command.Builder<C>, commandManager: CommandManager<C>)
  ```
- Making the `commandBuilder` variable public
- Adding the following extensions
  
  ```kotlin
  public fun <C : Any> Command.Builder<C>.toMutable(commandManager: CommandManager<C>): MutableCommandBuilder<C>
  
  public fun <C : Any> Command.Builder<C>.mutate(commandManager: CommandManager<C>, lambda: MutableCommandBuilder<C>.() -> Unit): MutableCommandBuilder<C>
  ```

cloud-kotlin/cloud-kotlin-extensions/src/main/kotlin/cloud/commandframework/kotlin/MutableCommandBuilder.kt

@@ -39,11 +39,22 @@ import kotlin.reflect.KClass
  * A mutable [Command.Builder] wrapper, providing functions to assist in creating commands using the
  * Kotlin builder DSL style
  *
+ * @property commandBuilder the command builder the mutate
+ * @property commandManager the command manager which will own this command
+ * @constructor Create a new [MutableCommandBuilder]
  * @since 1.3.0
  */
-public class MutableCommandBuilder<C : Any> {
+public class MutableCommandBuilder<C : Any>(
-    private val commandManager: CommandManager<C>
+    commandBuilder: Command.Builder<C>,
-    private var commandBuilder: Command.Builder<C>
+    private val commandManager: CommandManager<C>,
+) {
+    /**
+     * The command builder that is being mutated by this [MutableCommandBuilder] instance.
+     *
+     * This is public so that this can be returned to a command builder for interop with java apis.
+     */
+    public var commandBuilder: Command.Builder<C> = commandBuilder
+        private set
 
     /**
      * Create a new [MutableCommandBuilder]
@@ -64,10 +75,7 @@ public class MutableCommandBuilder<C : Any> {
         description: Description = Description.empty(),
         aliases: Array<String> = emptyArray(),
         commandManager: CommandManager<C>
-    ) {
+    ) : this(commandManager.commandBuilder(name, description, *aliases), commandManager)
-        this.commandManager = commandManager
-        this.commandBuilder = commandManager.commandBuilder(name, description, *aliases)
-    }
 
     /**
      * Create a new [MutableCommandBuilder]
@@ -83,10 +91,7 @@ public class MutableCommandBuilder<C : Any> {
         description: ArgumentDescription = ArgumentDescription.empty(),
         aliases: Array<String> = emptyArray(),
         commandManager: CommandManager<C>
-    ) {
+    ) : this(commandManager.commandBuilder(name, description, *aliases), commandManager)
-        this.commandManager = commandManager
-        this.commandBuilder = commandManager.commandBuilder(name, description, *aliases)
-    }
 
     /**
      * Create a new [MutableCommandBuilder] and invoke the provided receiver lambda on it
@@ -133,11 +138,6 @@ public class MutableCommandBuilder<C : Any> {
         lambda(this)
     }
 
-    private constructor(commandManager: CommandManager<C>, commandBuilder: Command.Builder<C>) {
-        this.commandManager = commandManager
-        this.commandBuilder = commandBuilder
-    }
-
     /**
      * Build a [Command] from the current state of this builder
      *
@@ -184,7 +184,7 @@ public class MutableCommandBuilder<C : Any> {
      * @since 1.3.0
      */
     public fun copy(): MutableCommandBuilder<C> =
-        MutableCommandBuilder(this.commandManager, this.commandBuilder)
+        MutableCommandBuilder(this.commandBuilder, this.commandManager)
 
     /**
      * Make a new copy of this [MutableCommandBuilder] and invoke the provided receiver lambda on it

cloud-kotlin/cloud-kotlin-extensions/src/main/kotlin/cloud/commandframework/kotlin/extension/CommandBuildingExtensions.kt

@@ -129,6 +129,28 @@ public fun <C : Any> CommandManager<C>.command(
 public fun <C : Any> Command.Builder<C>.senderType(type: KClass<out C>): Command.Builder<C> =
     senderType(type.java)
 
+/**
+ * Create a new [MutableCommandBuilder].
+ *
+ * @param commandManager the command manager, which will own this command.
+ * @since 1.7.0
+ */
+public fun <C : Any> Command.Builder<C>.toMutable(
+    commandManager: CommandManager<C>
+): MutableCommandBuilder<C> = MutableCommandBuilder(this, commandManager)
+
+/**
+ * Create a new [MutableCommandBuilder] and invoke the provided receiver lambda on it.
+ *
+ * @param commandManager the command manager, which will own this command.
+ * @param lambda receiver lambda, which will be invoked on the new builder.
+ * @since 1.7.0
+ */
+public fun <C : Any> Command.Builder<C>.mutate(
+    commandManager: CommandManager<C>,
+    lambda: MutableCommandBuilder<C>.() -> Unit
+): MutableCommandBuilder<C> = MutableCommandBuilder(this, commandManager).also(lambda)
+
 /**
  * Get a [Description], defaulting to [Description.empty]
  *