Implement string.gmatch iterator

[evadne]

Implements string.gmatch(s, pattern).

• This was previously a stub that always raised badarg
• This change implements the function using the existing pattern-matching engine.
• The implementation is pre-computed:
  • Pre-compute all matches eagerly via gsub_match_loop/6
  • Store the match list in luerl private state under a unique ref
  • Return an erl_func iterator that pops from the match list on each call.
  • The iterator state is cleaned up when the match list is exhausted.
  • Use luerl:put_private, get_private and delete_private to thread iterator state through the luerl state record rather than the process dictionary, keeping the implementation pure.
• The eager approach follows gsub, which also pre-computes all matches via gsub_match_loop/6, before processing replacements.
  • Both functions share the same match engine, so a future lazy stepper would benefit both equally.
• Added 9 new tests in luerl_str_gmatch_tests.erl.

Handles

• Basic iteration (word splitting, digit extraction)
• Multiple captures (key=value patterns)
• Single character matches
• Anchored patterns (^)
• No-match case (iterator immediately yields nil)
• Empty input string
• Multiple independent iterators
• Typical Lua book pattern: building tables from key=value strings

Should fix #150