Add "multisite" as new tab on the plugin install page on multisite installations

[zodiac1978]

Discussed this with @desrosj at WordCamp Europe 2026 Contributor Day:

We could add a new tab to the plugin install page if we are on a multisite installation.

Of course this is a decision of the plugin author to use this tag and it could be easily achieved with your own search, but this would require to set the dropdown to tag and type "multisite" yourself.

Offering an easy link to get all plugins tagged with "multisite" could help to find plugins that are helpful in this multisite environment.

[zodiac1978]

First try to add this new multisite tab for multisite installations

[zodiac1978]

Fixed unrelated code changes

[zodiac1978]

Second patch should work and add this new tab only on multisites.

Adds a new tab to the plugin install page if we are on a multisite installation.

Of course this is a decision of the plugin author to use this tag and it could be easily achieved with your own search, but this would require to set the dropdown to tag and type "multisite" yourself.

Offering an easy link to get all plugins tagged with "multisite" could help to find plugins that are helpful in this multisite environment.

Trac ticket: Core-65455

## Use of AI Tools

None.

[zodiac1978]

Thanks to @soean for catching the superfluous spaces and recommending using GitHub to get the automated tests.

Ready to review @desrosj @realloc