Support SE-0393: Parameter Packs

[patrickt]

This enhancement introduces a lot of new syntax. Right now the highlighter manages to recognize repeat but it has difficulty with each and friend.

func makePairs<each First, each Second>(
  firsts first: repeat each First,
  seconds second: repeat each Second
) -> (repeat Pair<each First, each Second>) {
  return (repeat Pair(each first, each second))
}

[jtbandes]

for comparison :)

```swift
func makePairs<each First, each Second>(
  firsts first: repeat each First,
  seconds second: repeat each Second
) -> (repeat Pair<each First, each Second>) {
  return (repeat Pair(each first, each second))
}

[alex-pinkus]

This looks like someone spilled a bowl of keyword soup on the floor lmao

[alex-pinkus]

I suppose if the swift folks want to make their language look like this, I have no choice but to be complicit