Structural handler arrays should be typed as const

[nicholaides]

RuleFoo
  Numeric Alpha -> [$1, $2]

current type: Array<number | string>
but we'd prefer: [number, string]

(Assuming Numeric returns a number and Alpha returns a string)

Same should apply to arrays that aren't top level.

RuleFoo
  Numeric Alpha -> { items: [$1, $2] }

current type: { items: Array<number | string> }
but we'd prefer: { items: [number, string] }