fix: interface_statement have decorator_list

Indeed, the web documentation makes it appear at first glance that interface does not have decorators,
but looking at the implementation, it does.
It is also shown in the example code.

<https://github.com/microsoft/typespec/blob/a939c140980805dc66a9b6fef70be5647b1c9680/packages/samples/specs/versioning/main.tsp#L51>

The PR to fix the standard and documentation has not yet been merged, but has already been submitted.

[docs: interface statement can have decorators by ncaq · Pull Request #5202 · microsoft/typespec](microsoft/typespec#5202)

I found it because of a problem that the parser here could not highlight the sample code correctly.

I'll submit a PR here first as well.

grammar.js

@@ -172,6 +172,7 @@ module.exports = grammar({
     scalar_extends: $ => seq("extends", $._expression),
 
     interface_statement: $ => seq(
+      optional($.decorator_list),
       "interface",
       field("name", $.identifier),
       optional($.template_parameters),