chore: Clippy fixes for 1.80 (#13987)

The biggest hurdle turned out to be use of `Arc<Language>` in maps, as
`clippy::mutable_key_type` started triggering on it (due to - I suppose
- internal mutability on `HighlightMap`?). I switched over to using
`LanguageId` as the key type in some of the callsites, as that's what
`Language` uses anyways for it's hash/eq, though I've still had to
suppress the lint outside of language crate.

/cc @maxdeviant , le clippy guru.

Release Notes:

- N/A

crates/project/src/task_inventory.rs

@@ -444,11 +444,6 @@ mod test_inventory {
 
     use super::{task_source_kind_preference, TaskSourceKind, UnboundedSender};
 
-    #[derive(Debug, Clone, PartialEq, Eq)]
-    pub struct TestTask {
-        name: String,
-    }
-
     pub(super) fn static_test_source(
         task_names: impl IntoIterator<Item = String>,
         updates: UnboundedSender<()>,