lint: Enable 'prefer-const' rule

Now that all code conforms with the rule, we can enforce it to make
sure we stick to it with new code.

Relax the rule for destructuring, so we don't have to jump through
hoops to avoid "mixed" assignments.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/432>

tools/eslint.config.js

@@ -20,6 +20,9 @@ export default defineConfig([
                 afterColon: true,
             }],
             'prefer-arrow-callback': 'error',
+            'prefer-const': ['error', {
+                destructuring: 'all',
+            }],
             'jsdoc/require-param-description': 'off',
             'jsdoc/require-jsdoc': ['error', {
                 exemptEmptyFunctions: true,