Add file system context action to add translation(s)

Right clicking a `.translation` file -> "ModTool: Add translation"

That adds
```gdscript
ModLoaderMod.add_translation("path/to/translation/resource/file.translation")
```
to the *mod_main.gd* `add_translations()` function.

Start by locating the handling for file-system context actions and the mod_main.gd add_translations() function. Trace how a selected .translation file is identified and how entries are added there. Done means the “ModTool: Add translation” action adds the selected resource path as a ModLoaderMod.add_translation call.

Written by the indexing model from the issue text.