Add support for logged out 404 pages

[gamemaker1]

Overview

When logged in and in the file tree, GitHub changes the page title to File not found if the requested file/folder is missing.

However, when logged out or an issue/pr is not found, the page title is changed to Page not found.

This PR updates the is404 function to reflect the above change.

Test URLs

• https://github.com/refined-github/refined-github/blob/main/source/features/a-horse-with-no-name.tsx
• https://github.com/refined-github/refined-github/blob/main/source/a-unicorn-with-no-name/
• https://github.com/refined-github/refined-github/issues/999999999999

Merging this PR will be the first step in fixing https://github.com/refined-github/refined-github/blob/main/source/features/useful-not-found-page.tsx :)

[fregante]

Note that logged out pages are generally not supported by Refined GitHub and we don't add extra code to support them. This PR can be an exception to that rule

[gamemaker1]

Should I add any tests? I didn't find any for the is404 function.

[fregante]

github-url-detection/test.ts

Line 71 in b6f7b8d

test('is404', t => {

[gamemaker1]

I think the last test will fail now since we also check for Page Not Found:

github-url-detection/test.ts

Line 81 in b6f7b8d

document.title = 'Page not found · Issue #266 · sintaxi/surge · GitHub';

[fregante]

The tests are not failing in the current PR

[gamemaker1]

Oh right lol