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/markdown/src/parser.rs

@@ -144,7 +144,6 @@ pub enum MarkdownTag {
 
     /// A footnote definition. The value contained is the footnote's label by which it can
     /// be referred to.
-    #[cfg_attr(feature = "serde", serde(borrow))]
     FootnoteDefinition(SharedString),
 
     /// A table. Contains a vector describing the text-alignment for each of its columns.