Match opaque result types per SE-0244

[jtbandes]

Documentation:

• https://docs.swift.org/swift-book/ReferenceManual/Types.html#ID616
• https://github.com/apple/swift-evolution/blob/master/proposals/0244-opaque-result-types.md

Opaque result types are used heavily in SwiftUI code, for example:

• https://github.com/ivanvorobei/SwiftUI
• https://github.com/apple-sample-code/SwiftUI-Tutorials

Since some is a keyword that pertains only to types, it can still be used as a regular variable name. So this PR adds a pattern for let/var declarations with types, so that the some pattern can be enabled only where appropriate.

func opaqueTypes() -> some View {}
struct Foo {
  let some: Int? = .some(42)
  var body: some View {}
}