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/
  clutter plus gio/glib/gobject/gtk/adw)
- drop the @girs/gnome-shell umbrella: it is still GNOME 50 and pulls
  St-18 and Meta-18 types that clash with the -51 packages
- types/: hand-written declarations for the Shell JS modules imported here
  (extension.js, prefs.js, main.js, panelMenu.js, popupMenu.js, slider.js),
  since upstream 51 types (51.0.0-alpha.1) are unpublished. PopupMenu
  open/close are typed with the 51 parameters object
- TypeScript 6.0.3 rather than 7: typescript-eslint requires <6.1.0 and
  ts-jest requires <7
- eslint 10.12, @eslint/js 10.0.1, eslint-plugin-jsdoc 65.3.4 and
  typescript-eslint 8.71.2

types/shell-globals.d.ts

@@ -0,0 +1,20 @@
+/**
+ * GNOME Shell runtime globals.
+ *
+ * The `global` object is created by the shell before an extension is loaded and
+ * has no `.gir`, so it is not covered by the generated `@girs/*-51` packages.
+ * Upstream ships these in `@girs/gnome-shell`, but its GNOME 51 build
+ * (51.0.0-alpha.1) is unpublished, and v50 pulls St-18/Meta-18 types that
+ * collide with the -51 packages. Declaring the few symbols we use keeps the
+ * project on GNOME 51 types and installable from the registry.
+ *
+ * @see https://gitlab.gnome.org/GNOME/gnome-shell/-/blob/main/js/ui/environment.js
+ */
+import type Shell from "@girs/shell-51";
+
+declare global {
+    /** Global shell object created by GNOME Shell on startup. */
+    const global: Shell.Global;
+}
+
+export {};