Make filemanager.config package part of stable API

[ascopes]

Update the classes in java-compiler-testing/src/main/java/io/github/ascopes/jct/filemanagers/config part of the stable API.

This involves just changing the @API annotation on each of the files in this package from reading status = Status.INTERNAL to status = Status.STABLE.

[JoePorterDev]

I'll take this if that's okay?

[sohel-79]

@JoePorterDev use intelliJ and with it's find and replace you can do it easily

[ascopes]

Assigned, @JoePorterDev. Thanks!