dart: Add support for documentation comments (#19592)

Closes #19590

Release Notes:

- N/A
---

I'm unable to test this because rebuilding Zed with the changes does not
seem to use the changes. If maintainers could let me know how to test
these changes I'd like to verify that this really fixes #19590.

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

extensions/dart/languages/dart/config.toml

@@ -1,7 +1,7 @@
 name = "Dart"
 grammar = "dart"
 path_suffixes = ["dart"]
-line_comments = ["// "]
+line_comments = ["// ", "/// "]
 autoclose_before = ";:.,=}])>"
 brackets = [
     { start = "{", end = "}", close = true, newline = true },