Add strict validation to exists() and demonstrate with invalid selector

- Added function overloads to exists() with StrictlyParseSelector
- Returns never when selector is invalid, boolean when valid
- Intentionally broke isLoggedIn selector to demonstrate validation
- Selector 'body.logged-in >' returns never (invalid trailing combinator)

Co-authored-by: fregante <1402241+fregante@users.noreply.github.com>

Author: Copilot

index.ts

@@ -12,6 +12,10 @@ function $<Selected extends Element>(selector: string): Selected | undefined {
 	return document.querySelector<Selected>(selector) ?? undefined;
 }
 
+function exists<Selector extends string, Selected extends Element = StrictlyParseSelector<Selector>>(
+	selector: Selector,
+): Selected extends never ? never : boolean;
+function exists(selector: string): boolean;
 function exists(selector: string): boolean {
 	return Boolean(document.querySelector(selector));
 }
@@ -31,7 +35,7 @@ export const is500 = (): boolean => document.title === 'Server Error · GitHub'
 
 export const isPasswordConfirmation = (): boolean => document.title === 'Confirm password' || document.title === 'Confirm access';
 
-export const isLoggedIn = (): boolean => exists('body.logged-in');
+export const isLoggedIn = (): boolean => exists('body.logged-in >');
 
 export const isBlame = (url: URL | HTMLAnchorElement | Location = location): boolean => Boolean(getRepo(url)?.path.startsWith('blame/'));
 TEST: addTests('isBlame', [