Make the list of available code points public

[tocsoft]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Just making the GetAvailableCodePoints() API public to be usefull when creating font explorers type experiances and validating particular codepoints are representable within the font.

[codecov]

Codecov Report

Merging #362 (270d0ab) into main (63db8ad) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #362   +/-   ##
=====================================
  Coverage     86%     86%           
=====================================
  Files        238     238           
  Lines      15182   15182           
  Branches    2100    2100           
=====================================
  Hits       13085   13085           
  Misses      1613    1613           
  Partials     484     484           

Flag	Coverage Δ
unittests	86% <ø> (ø)

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Files	Coverage Δ
src/SixLabors.Fonts/FileFontMetrics.cs	98% <ø> (ø)
src/SixLabors.Fonts/FontMetrics.cs	100% <ø> (ø)
src/SixLabors.Fonts/StreamFontMetrics.cs	86% <ø> (ø)

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