Support word-based completions (#26410)

Closes https://github.com/zed-industries/zed/issues/4957


https://github.com/user-attachments/assets/ff491378-376d-48ec-b552-6cc80f74200b

Adds `"completions"` language settings section, to configure LSP and
word completions per language.
Word-based completions may be turned on never, always (returned along
with the LSP ones), and as a fallback if no LSP completion items were
returned.

Future work:

* words are matched with the same fuzzy matching code that the rest of
the completions are

This might worsen the completion menu's usability even more, and will
require work on better completion sorting.

* completion entries currently have no icons or other ways to indicate
those are coming from LSP or from word search, or from something else

* we may work with language scopes more intelligently, group words by
them and distinguish during completions

Release Notes:

- Supported word-based completions

---------

Co-authored-by: Max Brunsfeld <max@zed.dev>

crates/proto/proto/zed.proto

@@ -1002,10 +1002,13 @@ message Completion {
     bool resolved = 6;
     Source source = 7;
     optional bytes lsp_defaults = 8;
+    optional Anchor buffer_word_start = 9;
+    optional Anchor buffer_word_end = 10;
 
     enum Source {
         Lsp = 0;
         Custom = 1;
+        BufferWord = 2;
     }
 }