Add StringIndex and GraphemeIndex to GlyphBounds

[gerdus]

Prerequisites

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Description

Add StringIndex and GraphemeIndex to GlyphBounds to enable hit testing glyphs and get the string index.

[CLAassistant]

All committers have signed the CLA.

[gerdus]

ref discussion: #386

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86%. Comparing base (15b7f8e) to head (5cfb1fd).
Report is 4 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #396   +/-   ##
=====================================
  Coverage     86%     86%           
=====================================
  Files        238     238           
  Lines      15229   15251   +22     
  Branches    2108    2108           
=====================================
+ Hits       13189   13219   +30     
+ Misses      1566    1561    -5     
+ Partials     474     471    -3     

Flag	Coverage Δ
unittests	86% <100%> (+<1%)	⬆️

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[JimBobSquarePants]

This looks great to me. Thanks!