zig: Account for doctests in outline (#19776)

zig has a feature called
[doctests](https://ziglang.org/documentation/master/#Doctests) where
instead of providing a string as the name of a test you use an
identifier so that the test is "tied" to it and can be used in
documentation. this wasnt accounted for so any tests using this were
unnamed in the outline

Release Notes:

- N/A

extensions/zig/languages/zig/outline.scm

@@ -19,6 +19,9 @@
 (
     TestDecl (
         "test" @context
-        (STRINGLITERALSINGLE)? @name
+        [
+         (STRINGLITERALSINGLE)
+         (IDENTIFIER)
+        ]? @name
     )
 ) @item