Add new "hide deprecated items" setting in rustdoc
This PR adds a new JS setting which allows the JS to hide deprecated items. This is especially useful for crates like `std` which accumulates deprecated items but never removes them.
Nicely enough, the "deprecation" information was already in the search index, meaning I didn't need to change anything there. Before this PR, it was only used to make the deprecated items rank lower. Well now it's also used to generate an extra CSS class, allowing the JS setting to work.
This is taking over https://github.com/rust-lang/rust/pull/149551.
This feature got approved by the rustdoc team in the [meeting of december](https://rust-lang.zulipchat.com/#narrow/channel/393423-t-rustdoc.2Fmeetings/topic/2025-12-08/near/562553156).
You can give it a try [here](https://rustdoc.crud.net/imperio/hide-deprecated-items/foo/index.html).
r? @lolbinarycat
When this fails, you get output that looks like:
/home/user/rust/tests/rustdoc-gui/search-result-impl-disambiguation.goml search-result-impl-disambiguation... FAILED
[ERROR] `tests/rustdoc-gui/utils.goml` around line 49
from `tests/rustdoc-gui/search-result-impl-disambiguation.goml` line 25: JS errors occurred: Event: Event
Making the error message more informative requires patching browser-ui-test.
