Relax visibility rules for type-aliases when 'declaration' compiler option is set.

One common thing I use type-aliases for is to give shorter, more expressive names to commonly used types throughout a file.

This is especially useful for callback signatures that see common use throughout a module, as well as string-literal types used in a union type, like type HttpMethods = 'get' | 'post' | 'put' | 'delete'.

Let's look at the following simplified example:

type CharCallback = (c: string, i: number) => string;

export function mapOnChar(str: string, fn: CharCallback): string {
  const newStr = [];
  for (let i = 0, lim = str.length; i < lim; i++)
    newStr.push(fn(str.charAt(i), i));
  return newStr.join('');
}

When using the declaration compiler option, this example fails to compile with the following error:
error TS4078: Parameter 'fn' of exported function has or is using private name 'CharCallback'.

This initially makes sense; the type-alias is not being exported so it is private. However, if you look at the type-alias, the only thing "private" about it is the name it was given, and an alias' name is not really important to or needed for a definition file.

There is no reason that the un-exported type-alias in the example cannot be automatically de-aliased back into (c: string, i: number) => string and that used in its place when the definition file is emitted.

However, if the alias in the example were to be exported, then it should not be de-aliased in the definition file.

I should point out that this suggestion is considering type-aliases only. If the CharCallback type was re-written as interface CharCallback { (c: string, i: number): string }, then that would be a good case to raise an error. An interface is generally considered more "concrete" than a simple type-alias, and so its name should be preserved and used in the definition file.

Start with the reported TypeScript example using an unexported CharCallback alias, an exported function, and the declaration compiler option. Trace declaration emission and the TS4078 visibility check; done means unexported aliases can be inlined in emitted declarations while exported aliases remain named, with the interface case still rejected.