Autocomplete docs (#3126)

Release Notes:

- Added documentation display for autocomplete items.
- Fixed autocomplete filtering blocking the Zed UI, causing hitches and
input delays with large completion lists.
- Fixed hover popup link not firing if the mouse moved a slight amount
while clicking.
- Added support for absolute path file links in hover popup and
autocomplete docs.

crates/language/src/language.rs

@@ -2,6 +2,7 @@ mod buffer;
 mod diagnostic_set;
 mod highlight_map;
 pub mod language_settings;
+pub mod markdown;
 mod outline;
 pub mod proto;
 mod syntax_map;