Allow referencing items from a template parameter

[timotheeguerin]

fix #1280
Revisit #1543

This is quite key to making option bag for template a reality.

alias Options = {
  name: string;
  value: string;
};
model Foo<T extends Options> {
  t: T.name;
}

Limitations: You have to use an alias. Cannot do it inline.

Todo:

• tests

[github-actions]

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Website: https://cadlwebsite.z1.web.core.windows.net/prs/2101/

[timotheeguerin]

Right now when we resolve a template declaration type (uninstantiated) and you reference a template parameter with constrain the reference remains the template parameter but with this it would become the type in the constraint.

alias Options = {name: string};

model Foo<T extends Options> {
  n: T.name; // This resolve to `Scalar: string`
  t: T;            // This resolve to `TemplateParameter: T`
}

Wondering if then we need a new Type TemplateParameterMember or something like that to represent that accurately and not loose information