Generate lang files at build time

[MattSturgeon]

• [x] Refactor :metadata into a more general :data project
• [x] Introduce a custom gradle task that can "build" lang files
• [x] Move most VariantTooltip logic to the gradle task
• [x] Move mod description to lang files
• [x] Use the built description in mod metadata
• [ ] Reformat lang files as .properties (not planned)
• [ ] Split lang files into main & config (low priority?)
• [ ] Allow defining per-file key prefix/suffix or json "flattening" (not planned)
• [x] Fabric support (using ModMenu's documented API)
• [ ] Forge support (depends on https://github.com/neoforged/NeoForge/issues/639)
• [ ] Unit tests for LangTask
• [x] Unit tests for the LangProcessor implementations

Fixes #172

[MattSturgeon]

This is feature complete and ready for review.

General notes

• First time using Kotlin; seems nice so far. A very expressive language, and excellent null safety.
  • Kotlin was used to experiment with what it's like, with a view to potentially porting other parts of the build script in future.
• I would have liked to implement functional tests using Gradle's TestKit, however I was running into issues getting buildSrc into the runner's classpath...
  • I may still add some unit tests though.
• buildSrc is an entirely separate "build", with it's own gradle "projects".
  • Gradle automatically compiles buildSrc before running the main build, and makes the result available to the build script.
  • It is a simple way to have freecam-specific build logic, plugins, & tasks without having to clutter up the actual build script.
• I've verified that :data builds everything correctly.

Tooltips

• VariantTooltip is now mostly a build step, the only thing handled at runtime is line count.
• Mod name and description are now handled by :data's langTask.
  • If there is a freecam.name.<variant> translation, it is appended to the end of freecam.name.
  • Same for description.
  • These are also used for the name/description in the mod metadata file.
  • These are also assigned to the various modmenu-specific translation keys.

Forge

• Forge's Mod Menu doesn't support translating mod info
• I patched in support via two mixins.
  • My gut feeling says this is a bad idea, and we should instead open a NeoForge issue/PR.
  • I've opened https://github.com/neoforged/NeoForge/issues/639, let's see if there's any interest.
  • The forge commit can be dropped from this PR if we'd rather stick to native behavior.

[MattSturgeon]

Added some basic unit tests for the LangProcessor implementations & cleaned up LangTask a little.

[MattSturgeon]

Dropped the forge mixin for now, hopefully we'll get a proper upstream solution.

[MattSturgeon]

Neoforge has added support in v2.4.179, using the following keys:

• fml.menu.mods.info.displayname.freecam
• fml.menu.mods.info.description.freecam

I've also been thinking about better ways to approach the transformer funcs and "variant" translations, at risk of making this even more over-engineered.