Allow to name the constants of a string union by configuration

[Vampire]

This PR is a follow-up to #22, only the top-most commit is to be reviewed.

With this PR you can manually map string union values to identifier names
using the Karakum configuration file.

[sgrishchenko]

I like the idea, but I am afraid of cases when you have two enums with the same name but in different packages.
I would propose another strategy. Let’s introduce a new extension point with signature: (node: TNode, context: ConverterContext) => string | null (like NameResolver). With this approach you will have full control over AST, and you can select different strategies for renaming even if you have multiple enums with the same name.

[Vampire]

Probably better, yeah, the other just seemed easier. :-D
Well, in the end it was not really.
See updated code :-)

[Vampire]

Superseded by bbab142