Fix some warnings/issues uncovered by the new cfg checking (#12310)

Rust recently got the ability to check for typos or errors in `cfg`
attributes: https://blog.rust-lang.org/2024/05/06/check-cfg.html
This PR fixes the new warnings.

- gpui can be run with `RUSTFLAGS="--cfg gles"`, make this explicit in
`[workspace.lints.rust]`
- `cfg!(any(test, sqlite))` was just a bug, it should be
`feature(sqlite)`
- the `languages` crate had a `#[cfg(any(test, feature =
"test-support"))]` function without ever declaring the `test-support`
feature
- the `MarkdownTag` enum had a `cfg_attr` for serde without actually
having serde support


Now the only warnings when building are unused fields
`InlayHover.excerpt`, `SavedConversationMetadata.path` ,
`UserTestPlan.allow_client_reconnection` and `SyntaxMapCapture.depth`.

Release Notes:

- N/A

crates/collab/src/db/queries/channels.rs

@@ -713,7 +713,7 @@ impl Database {
                     .find_also_related(user::Entity)
                     .filter(channel_member::Column::ChannelId.eq(channel.root_id()));
 
-                if cfg!(any(test, sqlite)) && self.pool.get_database_backend() == DbBackend::Sqlite {
+                if cfg!(any(test, feature = "sqlite")) && self.pool.get_database_backend() == DbBackend::Sqlite {
                     query = query.filter(Expr::cust_with_values(
                         "UPPER(github_login) LIKE ?",
                         [Self::fuzzy_like_string(&filter.to_uppercase())],