No Parser.backtrackable

[Janiczek]

This function is an antipattern and becomes inefficient when used together with Parser.oneOf. The thing is, it's a quick fix sometimes and leads to nicer-looking (more declarative) code, so people are tempted to use it.

Disallow usages of this function.

Note there are two variants: Parser.backtrackable and Parser.Advanced.backtrackable.

BONUS ROUND: have a fix that rewrites the code from backtrackable to non-backtrackable. (Likely hard? Spread across multiple declarations etc.)