language: Accept multiple values in line_comment language knob. (#6713)

This opens up a possibility of supporting multiple comment continuation
flavours in editor, e.g. doc comments for Rust (which we seize as well
in this commit). Only the first `line_comment` value is used for
Editor::ToggleComments

Fixes: https://github.com/zed-industries/zed/issues/6692

Release Notes:
- Added support for doc-comment continuations in Rust language.

crates/zed/src/languages/cpp/config.toml

@@ -1,6 +1,6 @@
 name = "C++"
 path_suffixes = ["cc", "cpp", "h", "hpp", "cxx", "hxx", "inl"]
-line_comment = "// "
+line_comments = ["// "]
 autoclose_before = ";:.,=}])>"
 brackets = [
     { start = "{", end = "}", close = true, newline = true },