Add OWML mod skeleton and import English translation template

Phase 2 (mod skeleton) + Phase 3 (import English source):

- manifest.json / SlovakTranslation.cs / ILocalizationAPI.cs registering
  "Slovenčina" via xen.LocalizationUtility (Interplanetary Polyglot).
- SlovakTranslation.csproj / .sln building net48 against the OWML and
  OuterWildsGameLibs NuGet packages — no game install needed to compile.
- assets/Translation.xml: English source template (896 KB, 2424 entries,
  keys == values) to be translated into Slovak.
- Project docs (README/PLAN/CLAUDE) and .gitignore.

Build verified: dotnet build -c Release stages a loadable mod folder.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,11 @@
+# Build output
+bin/
+obj/
+
+# User-specific project files
+*.user
+
+# IDE
+.vs/
+.vscode/
+.idea/