Make sure to preserve a space between `?` and `.`

[munificent]

We're working on two new language features:

• Null-aware elements
• Static access shorthands

These two features can be combined:

var list = <double>[? .infinity];

It's sort of like we're adding two new prefix operators (though the . isn't really a prefix operator). However, there is already a ?. operator and the lexical grammar tokenizes that as a single unit.

This means that if the formatter encounters a shorthand .foo expression inside a null-aware element ? expression, it must preserve a space between the ? and .. We have a similar edge case in the formatter around preserving a space between chained unary - operators like - -a so that it doesn't accidentally turn them into a prefix decrement.