feat(annotations): add @CommandMethod annotation processing (#366)

We now verify the following at compile time:
- That `@CommandMethod` annotated methods are non-static (error)
- That `@CommandMethod` annotated methods are public (warning)
- That the `@CommandMethod` syntax and specified `@Argument`s match
- That no optional argument precedes a required argument

cloud-annotations/src/test/resources/TestCommandMethodMissingArgument.java

@@ -0,0 +1,11 @@
+import cloud.commandframework.annotations.Argument;
+import cloud.commandframework.annotations.CommandMethod;
+
+public class TestCommandMethodMissingArgument {
+
+    @CommandMethod("command <required> [optional]")
+    public void commandMethod(
+            @Argument("optional") final String optional
+    ) {
+    }
+}