Add a quick way to format 'until' in toText

[LosD]

It's not the most beautiful of ways, essentially adding an optional argument to RRule.toText, but with NLP being split out, it seemed like an okay stopgap for now. I'd have preferred adding a method to the Language interface, but I couldn't without either breaking existing Language implementations (compile-time only, though), or making a new sub interface. This seemed a bit cleaner.

Fixes #312

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[codecov-io]

Codecov Report

Merging #313 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
+ Coverage   89.87%   89.88%   +0.01%     
==========================================
  Files          28       28              
  Lines        1945     1947       +2     
  Branches      581      581              
==========================================
+ Hits         1748     1750       +2     
  Misses        197      197

Impacted Files	Coverage Δ
src/rrule.ts	98.98% <100%> (ø)	⬆️
src/nlp/index.ts	100% <100%> (ø)	⬆️
src/nlp/totext.ts	95.85% <100%> (+0.03%)	⬆️

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[davidgoli]

Looks good to me, thanks for the contribution!