[vscode] Syntax highlighting breaks with (_ [@bs.as "foo"]) in external

Using `(_ [@bs.as "foo"])` as part of an external declaration breaks further syntax highlighting:

Adding `(* "\"" *)` right after the problematic `external` declaration returns things to normal.

Reproduce the issue in VS Code with an external declaration containing `(_ [@bs.as "foo"])`, then inspect the syntax-highlighting handling for this construct. Done means highlighting continues correctly after the declaration without requiring the `(* "\"" *)` workaround.

Written by the indexing model from the issue text.