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

.checkstyle/checkstyle-suppressions.xml

@@ -5,4 +5,5 @@
 <suppressions>
     <suppress checks="(?:(?:Member|Method)Name|DesignForExtension|Javadoc.*)" files=".*[\\/]mixin[\\/].*"/>
     <suppress checks="(?:Javadoc.*)" files=".*[\\/]bukkit[\\/]internal[\\/].*"/>
+    <suppress checks="(?:Javadoc.*)" files=".*[\\/]example-.*[\\/].*"/>
 </suppressions>