fix: interface_statement have decorator_list (#8)

* fix: Allow `interface_statement` to 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.

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),