[6.x] Fix taxonomy index terms for multi-site and Blade

[joshuablum]

This PR fixes two problems with $terms on taxonomy index routes (e.g. /tags):

1. 500 on multisite with the Eloquent driver

Taxonomy::toResponse() passed the current Site object to where('site', ...). Stache casts it to a string when comparing, so it worked there. The Eloquent driver passes the value to Term::in(), though, so the Site object ended up being used as a localization array key:

Cannot access offset of type Statamic\Sites\Site in isset or empty

• The taxonomy route now passes $site->handle(), like every other term query in core.
• The CP terms listing gets the same change (Site::selected()->handle()). It didn't crash, but only because it re-localizes the terms afterwards.
• Term::in() now accepts a Site instance and converts it to its handle, the same way Entry::locale() already does. This covers other code that passes a Site, on any driver.

2. @foreach ($terms as $term) renders nothing in Blade

$terms was a raw query builder. Query builders aren't iterable, so the Blade loop from the docs ran zero times. $terms (and the $tags-style alias) is now wrapped in Statamic\Fields\Value, which is already how query builders show up in augmented data, e.g. $entries on term pages. Iterating a Value runs the query, and method calls are forwarded to the builder.