Fix KT-78532 Appendable/StringBuilder objcExportErrorOnNameCollisions errors

[taiyrbegeyev]

KT-78532

Introduce @ObjCExportIgnoreNameCollision annotation to suppress name collision errors when generating Objective-C headers with -Xbinary=objcExportErrorOnNameCollisions=true compiler option.

Appendable and StringBuilder have multiple overloaded methods with different parameter types (append(Int), append(String), etc.) that cause mandatory name mangling in ObjC. This prevents users from enabling strict collision checking even when these types only appear in their API signatures without being used from ObjC.

Apply the annotation to Appendable and StringBuilder to allow these stdlib classes to work with the strict collision mode.

[taiyrbegeyev]

@sbogolepov I've implemented this as a general annotation users can apply. Is this the intended scope, or should it be restricted to stdlib symbols only?