feat: null element support (POC)

[mdjastrzebski]

Summary

Test plan

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.25000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 93.58%. Comparing base (d218336) to head (264d123).

Files with missing lines	Patch %	Lines
src/constructs/quantifiers.ts	66.66%	3 Missing ⚠️
src/encoder.ts	84.61%	2 Missing ⚠️
src/builders.ts	83.33%	1 Missing ⚠️
src/constructs/capture.ts	75.00%	1 Missing ⚠️
src/constructs/char-class.ts	88.88%	1 Missing ⚠️
src/constructs/lookahead.ts	75.00%	1 Missing ⚠️
src/constructs/lookbehind.ts	75.00%	1 Missing ⚠️
src/constructs/negative-lookahead.ts	75.00%	1 Missing ⚠️
src/constructs/negative-lookbehind.ts	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #104      +/-   ##
===========================================
- Coverage   100.00%   93.58%   -6.42%     
===========================================
  Files           17       17              
  Lines          160      187      +27     
  Branches        42       55      +13     
===========================================
+ Hits           160      175      +15     
- Misses           0       12      +12     

Flag	Coverage Δ
	93.58% <81.25%> (-6.42%)	⬇️

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