Revise grammar and syntax model for scoped

[jcouv]

TL;DR: From LDM 7/27, we should write the language spec grammar for ref locals, ref fields, and scoped. Then we should consider how that affects our syntax model.

LDM expressed the desire to not put scoped into modifiers buckets, as modifiers can generally be re-ordered.
Instead scoped should be tied to the type. One proposal is to introduce type_modifiers following modifiers to hold scoped, ref and readonly in specific configurations.
This should take inspiration from Rex's spec draft for ref locals: dotnet/csharpstandard#213

Once that is done, we need to decide how to adjust our implementation.

1. do we want to keep scoped in modifiers, or can we stick it into RefType (extending its meaning to "type with type modifiers"), or maybe introduce a new ScopedType?
2. do we want to make use of that syntax model for new scenarios in parameters? (parameters don't currently make use of RefType and instead stick everything into modifiers for backcompat reasons)

Relates to test plan #59194

[cston]

See dotnet/csharplang#6338 for proposed grammar.
See #61647 for ScopedTypeSyntax for locals, and for decision to use ParameterSyntax.Modifiers for scoped in parameters.