feat: upgrade to gnome 51

Adds GNOME Shell 51 to shell-version and moves the build onto the
GNOME 51 type packages.

- switch to @girs/gjs 5.5.0 and the -51 namespace packages (shell/st/meta
  plus gio/glib/gobject/gtk/adw)
- drop @girs/gnome-shell 49, the pinned @girs/gjs 4.0.0-beta.38 and the 16
  pnpm overrides it needed. The umbrella is still GNOME 50 and pulls St-18
  and Meta-18 types that clash with the -51 packages
- types/: hand-written declarations for the two Shell JS modules this
  extension imports (extension.js and prefs.js). Upstream has these for 51
  in 51.0.0-alpha.1, but that is not published to npm yet, so declaring the
  handful of symbols we use keeps installs registry-only
- TypeScript 6.0.3 rather than 7: typescript-eslint requires <6.1.0 and
  ts-jest requires <7. TS 6 also needs an explicit rootDir (TS5011)
- eslint 10.12, @eslint/js 10.0.1, eslint-plugin-jsdoc 65.3.4 and
  typescript-eslint 8.71.2

metadata.json

@@ -5,6 +5,8 @@
   "shell-version": [
     "47",
     "48",
-    "49"
+    "49",
+    "50",
+    "51"
   ]
 }