Add Vanilla Tweaks auto-install from extras/datapacks/

Place datapacks_<ver>.zip or crafting_tweaks_<ver>.zip in extras/datapacks/
before running the Minecraft installer and they will be detected automatically.

- Pattern match: *datapack* → unzip into datapacks-download/; *craft* → copy as-is
- Version extracted from filename; compared against selected MC version at picker step
- Early banner notice when ZIPs are absent (where to put them and naming convention)
- If ZIPs present: show found files + version check, offer "Use pre-packaged VT?" (default y)
- Accepting skips the individual datapack picker entirely
- Manual fallback instructions updated to hint at extras/datapacks/ for next time
- Removed broken VT share# links from manual instructions
- Added extras/datapacks/README.md (naming convention + download sources)
- Added .gitignore: extras/datapacks/*.zip never committed

https://claude.ai/code/session_019XgsQ13XKm4Zj3cNsDNwHj

.gitignore

@@ -0,0 +1,2 @@
+# Vanilla Tweaks ZIP files placed for automatic install — never commit these
+extras/datapacks/*.zip