Type spreads of regular variants in patterns

[zth]

Solves part of #6273
Closes #6562
Closes #6277

This PoC adds support for type spreads of regular variants in patterns. This is already a thing with polyvariants, and now it's also a thing for regular variants.

Rationale is we already have variant spreads and coercion, which lets you go from small type to big type. Having this will let you go from big type to small type. This is a missing piece.

Specification:

• Any variant X that is a subtype of variant Y can be spread in a pattern matching of a value of type Y
• Using an alias ...subtypeVariant as value will give the type subTypeVariant to value
• Syntax: Spreads of regular variants are done like spreads of polyvariants, but without the leading #: | ...subtypeVariant as value => doSomethingWithSubTypeVariant(value)

type a = One | Two | Three
type b = | ...a | Four | Five

let doWithA = (a: a) => {
  switch a {
  | One => Js.log("aaa")
  | Two => Js.log("twwwoooo")
  | Three => Js.log("threeeee")
  }
}

let lookup = (b: b) =>
  switch b {
  | ...a as a => doWithA(a)
  | Four => Js.log("four")
  | Five => Js.log("five")
  }

Should be a well isolated feature, since it's dependent on the parser adding an attribute when the spread is a regular variant (no leading #) and no regular pre-existing code path is touched.

[zth]

@cristianoc did you see this here? That we're not caring about type params. Type params aren't allowed in spreads anyway right now so maybe it doesn't matter.

[cristianoc]

Maybe worth checking that there are no type params, so it's not overlooked in future.