Expose a TS API for adding extension fields to a schema in a custom decorator

[Veetaha]

Clear and concise description of the problem

This issue is based on a discussion #3328 and @timotheeguerin's suggestion to solve my particular use case. See details in that discussion. The gist is that I'd like to write a custom decorator that simplifies the following notation:

@extension("x-foo", Json<"value here">)

to the this one

@foo("value here")

Right now it's implemented in my code with a hack like this, which creates a "fake" template instantiation of Json<T> model.

import { createRekeyableMap } from "@typespec/compiler/utils";
import { $extension } from "@typespec/json-schema";

/**
* @param {import("@typespec/compiler").DecoratorContext} context
* @param {import("@typespec/compiler").Type} target
* @param {import("@typespec/compiler").StringLiteral} value
*/
export function $foo(context, target, value) {
   $extension(context, target, "x-foo", {
       kind: "Model",
       name: "Json",
       namespace: { name: "JsonSchema" },
       properties: createRekeyableMap([["value", { type: value }]])
   })
}

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[markcowl]

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