fix: `textDocument/documentHighlight`

[vekatze]

before:

define my-func(): type {
       ^^^^^^^^^^^^^^^^^^^^^ (highlighted)
  type
}

define main(): unit {
  let _ = this.loc-{*}test.my-func in // {*}: cursor
  print("Hello, world!\n")
}

after:

define my-func(): type {
       ^^^^^^^ (highlighted)
  type
}

define main(): unit {
  let _ = this.loc-{*}test.my-func in // {*}: cursor
  print("Hello, world!\n")
}