Add support for multi-word snippet prefixes (#42398)

Supercedes #41126

Closes #39559, #35397, and #41426

Release Notes:

- Added support for multi-word snippet prefixes

---------

Co-authored-by: Agus Zubiaga <hi@aguz.me>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
Co-authored-by: Cole Miller <cole@zed.dev>

crates/editor/src/code_completion_tests.rs

@@ -305,6 +305,8 @@ impl CompletionBuilder {
             icon_path: None,
             insert_text_mode: None,
             confirm: None,
+            match_start: None,
+            snippet_deduplication_key: None,
         }
     }
 }