chore: reorganize snippets into subdirectories

- move root gitignores into gitignores/ subdirectory
- add gitignores for dart, dotnet, go, gradle, java, jetbrains, maven, nextjs, node, sublimetext, vscode, windows, xcode
- add root .gitignore excluding .claude/settings.local.json
- add empty README.md and licences/ placeholders (Apache, MIT, Mozilla)

gitignores/Linux.gitignore

@@ -0,0 +1,7 @@
+### Linux
+*~
+.fuse_hidden*
+.directory
+.Trash-*
+.nfs*
+nohup.out