Record `installURL` and normalize update logic

[ACTCD]

https://github.com/quoid/userscripts/issues/248#issuecomment-1421925648

The order of checking for updates should be:

• @updateURL (optional-metadata/.meta.js)
• @downloadurl (optional-metadata/.user.js)
• installURL (optional/NOT-metadata/.user.js)

installURL is not metadata, but the real URL when installed from a remote URL. If @installURL metadata exists, it should be ignored.

If none of the three exists, it is considered a local script and cannot be checked for updates.