make @Language internal

[aSemy]

• make @Language internal, to avoid clashes with other libraries (like KxS JSON, which exposes their own implementation of @Language with a @RequiresOptIn guard)
• Moved @Language to a new directory (while keeping the package the same) - purely because I think the project layout is neater
• Removed old TODO

Related issues:

• https://youtrack.jetbrains.com/issue/KTIJ-16340/
• Convert to Kotlin JetBrains/java-annotations#34 (comment)
• decodeFromString should annotate string parameter with @Language Kotlin/kotlinx.serialization#2166

[alllex]

Visibility change and clean up is good, but I prefer to keep production classes well-structured with respect to the packages.

I moved the class to a directory corresponding to its package.

[aSemy]

Visibility change and clean up is good, but I prefer to keep production classes well-structured with respect to the packages.

I moved the class to a directory corresponding to its package.

I understand, but be aware that it might cause some problems (e.g. on Android) if there are two classes with the same name & dir.