feat(parser, compiler): add parsing a type reference from a string

[lrlna]

To parse standalone source text like [Type]!, for example for the federation directive @fields(type:).

closes #715

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Also Also Also, should this just be parsing into a Type directly instead of into a FieldType wrapper? A type reference doesn't have to be in a field, field/directive argument types are the same.

[SimonSapin]

Agree on both points: this can use ast::Type without introducing a FieldType struct, and the Ok result doesn’t need to contain diagnostics (or conversely, return Err if any diagnostic)

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It accepts [[String] (unclosed List) 🤔

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@SimonSapin Ready for review again I think!