suggestion: LiteralUnion

[xDivisionByZerox]

Description

I recently watched your video on "Fix the Fetch Type" and noticed that you implemented the MIME types for the "Content-Type" and "Accept" headers as a union of MIME types. While this approach is effective, you mentioned during the creation of the "TypedHeaders" type that you appreciate having auto-completion for some headers while allowing custom ones to be present as well.

Therefore, I wanted to bring to your attention that a similar approach can be applied to strings as well.

Please feel free to close this PR if you are already aware of this solution.

Explanation

Firstly, I would like to clarify that this is not my original work but was discussed (and solved) in a TypeScript issue back in 2019: microsoft/TypeScript#29729 (comment).
The linked issue-comment also provides an explanation of why/how this works:

In this code-snippet the value will be of type Suggestion | (string & {}) because though the union type Suggestion inherit string, it does not inherit string & {}, which means Suggestion and string & {} are treated as distinguishable types.