Add instantiate typekit

Use case #7469 and https://github.com/microsoft/typespec/discussions/9729

There is a few ways we could consider this.
```ts
// 1. Evaluate expr
$.type.instantiate("Array")

// 2. Pass args manually
$.type.instantiate($.type.resolve("Array"), [$.type.resolve("Foo")])
```

2. Is probably needed for more control but it is also right now blocked as `$.type.resolve("TemplateDecl")` won't resolve it, need a flag to allow resolving the declaration