Fork with completion support

Hi all,

I've created a fork which adds completion support to the library, which allows providing as-you-type autocomplete or TAB-completion experiences: https://github.com/jchapuis/scala-parser-combinators.git

In a nutshell, additions are:

• def completions(in: Input): Completions method which allows querying the list of possible completions for a certain input. This returns a structured type which can contain tagged and ranked completions.
• implementations of this method for all the combinators
• a new set of operators which allows tagging and ranking elements of the grammar

I have also implemented fuzzy matching completion parsers which are not present in the fork right now but that I could potentially integrate.

Would you consider this as a possible PR? It's of course extending the "traditional" functionality of parser combinators, so it add extra complexity and testing requirements that are maybe not desirable for the core library. At any rate I'm creating this issue also to to point out that building completion experiences using parser-combinators is feasible and actually works well with the functional nature of these grammars.

Start by reviewing the proposed fork at https://github.com/jchapuis/scala-parser-combinators.git and its completion-support changes. Assess the combinator implementations, tagging and ranking operators, and available tests before determining whether the feature fits the core library. Done requires an agreed scope and a defined testing plan for completion behavior.