Support for co-dependently conditionally typed arguments

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ, especially the "Common Feature Requests" section: https://github.com/Microsoft/TypeScript/wiki/FAQ --> ## Search Terms - "co dependent arguments" - "co-dependent arguments" - "codependent arguments" - "codependently arguments" - "codependently typed arguments" <!-- List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily --> ## Suggestion It should be easier to conditionally type a function argument based on the type/value (in the case of enums / string literal types) of a previous argument in the function. Having a more specific way to do this than currently possible should improve editor intelligence and performance. <!-- A summary of what you'd like to see added or changed --> ## Use Cases It is quite common to have an API where the type of the second argument depends on the type and/or value of the first. For example, take a look at the following JavaScript code. (It's a fairly trivial example, but it should illustrate the use case. ```js function reducer(actionType, args) { switch (actionType) { case "add": console.log(args.a + args.b); break; case "concat": console.log(args.firstArr.concat(args.secondArr)); break; } } reducer("add", { a: 1, b: 3 }); // 4 reducer("concat", { firstArr: [1,2], secondArr: [3, 4] }); // [1,2,3,4] ``` It is currently possible to type this, but it is extremely convulted and the editor somewhat struggles to understand this at the call site. This method is based on the fantastic article over at https://artsy.github.io/blog/2018/11/21/conditional-types-in-typescript/ ```ts enum ActionTypeEnum { add, conca…