rust: Highlight function signatures as functions (#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.

1 files changed, 3 insertions, 0 deletions

diff --git a/runtime/queries/rust/highlights.scm b/runtime/queries/rust/highlights.scm
index 7d0afcde..81f05f7a 100644
--- a/runtime/queries/rust/highlights.scm
+++ b/runtime/queries/rust/highlights.scm
@@ -253,6 +253,9 @@
 (function_item
   name: (identifier) @function)
 
+(function_signature_item
+  name: (identifier) @function)
+
 ; ---
 ; Macros
 ; ---