Add a proposal for a string interpolation default parameter

[natecook1000]

A new string interpolation syntax for providing a default string when interpolating an optional value.

let age: Int? = nil
print("Your age: \(age, default: "missing")")
// Prints "Your age: missing"

[xwu]

@natecook1000 Happy to kick off review whenever—if you'd like a chance to tack on a paragraph about Either as an alternative/future direction, I can wait for that. Let me know!