rust: Highlight function signatures as functions (helix-editor#4073)

This stanza highlights functions within trait definitions. For example,
in:

    pub trait Widget {
        fn render(self, area: Rect, buf: &mut Buffer);
    }

`render` is currently highlighted as a variable. With this change it's
highlighted as a function.

runtime/queries/rust/highlights.scm

@@ -253,6 +253,9 @@
 (function_item
   name: (identifier) @function)
 
+(function_signature_item
+  name: (identifier) @function)
+
 ; ---
 ; Macros
 ; ---