broken @as in external does weird things

i just inlined a small binding in a file:

module Node = {
  module Fs = {
    @module("node:fs") external existsSync: string => bool = "existsSync"
    @module("node:fs")
    external readFileSync: (string, @as(json`"utf8`) _) => string = "readFileSync"
  }
}

i missed the closing " after utf8. having format on save, the formatter did a weird change:

    external readFileSync: ((string, @as(json`"utf8`) _)) => string = "readFileSync"

it added a double set of parens. this is a really edge case scenario so you may throw this in the trash if you like

Start by reproducing the formatter behavior with the external binding and malformed @as(json"utf8) snippet shown in the issue. Trace the formatter handling for external declarations and @as syntax; done means formatting this input no longer adds the extra pair of parentheses.