Record `installURL` and normalize update logic

[ACTCD]

#248 (comment)

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.