enhance: rewrite regex generator in TypeScript

[kakkokari-gtyih]

Problem

As discussed in #2, migrating regex generator to Flow (JavaScript) is a long-term goal for this project. Regex generator is written in Scala, which makes it harder to contribute, especially by JS/TS ecosystem developers.

Solution

This pull request rewrites the regex generator from Scala to TypeScript.

At this moment, I am not working on converting other parts or modernizing the build steps (e.g. get rid of Mustache). However, if that's a better option, I can work on that too (depending on my personal schedule, though)

Result

The generated regex is identical to the current latest version (check that there are no differences in the regex part of regex.js).

[kakkokari-gtyih]

I misread #2 and mistakenly thought the goal was to rewrite the regex generator in TypeScript (it is actually JS+Flow), my bad.

I rewrote it in TypeScript in this PR, but how should I proceed? Should I rewrite it with flow?