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/languages/Cargo.toml

@@ -8,6 +8,9 @@ license = "GPL-3.0-or-later"
 [lints]
 workspace = true
 
+[features]
+test-support = []
+
 [dependencies]
 anyhow.workspace = true
 async-compression.workspace = true