feat(intl): pass unique index to rich tag handlers to use as framework key

[dangreen]

Rich tag handlers now receive an unique index as the second argument, usable as a key for framework nodes (e.g. React), so consumers no longer need Children.toArray workarounds.

Also makes mapTags reentrant: nested calls from tag handlers no longer clobber the shared regex state of the outer call.

Built on top of #185.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.96%. Comparing base (fa4fcb8) to head (4fc8007).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   83.95%   83.96%   +0.01%     
==========================================
  Files         138      138              
  Lines        2985     2988       +3     
  Branches      564      564              
==========================================
+ Hits         2506     2509       +3     
  Misses        340      340              
  Partials      139      139              

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