vscode: Use tabs per default, XDG & gitignore

Use tabs instead of spaces per default. (VSCodium will still respect the
indentation used in the file)
Stop vscode from writing files in HOME.
Since only the settings.json is versioned in this repo but the full
folder is symlinked: Ignore all other files that will be written.
(TODO: Only symlink file)

.gitignore

@@ -13,6 +13,9 @@
 # Ignore installation backups
 /existing-*.tar.gz
 
+/.config/VSCodium
+!/.config/VSCodium/User/settings.json
+
 /.config/git/github.config
 /.config/git/gitlab.config
 /.config/git/user.config