Resource importer for FTL files + Better API

[RedMser]

Check the CSV importer for reference on how to implement this.

• ResourceFormatLoader: works in game and in editor. Could load .ftl files at runtime.
• EditorImportPlugin: only works in editor.
  • Variant 1: Saves .translation files like CSV, which would lose FTL data (only keeping messages with empty args) -> this approach does not make sense.
  • Variant 2: Store AST for quicker reconstruction of FTL data (see Investigate binary AST storage #6).

Ideally it should be possible to have multiple FTL files for one language, which can be merged. But neither a requirement, nor am I sure how Godot currently handles multiple Translations for the same locale. Would need to check for conflicting entries as well, or maybe adding multiple bundle files to the same Translation resource...

API should be improved e.g. by removing redundant language ID.