Add astc files so they are checked in with LFS

[Erik-White]

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

Adding ASTC compression support in ImageSharp.Textures requires checking in some .astc test data files. This change just ensures that astc files are checked in with LFS and prevents changes to the attributes file in ImageSharp.Textures from being overwritten

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 19%. Comparing base (8a9aa0c) to head (7596697).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@        Coverage Diff         @@
##           main   #56   +/-   ##
==================================
  Coverage    19%   19%           
==================================
  Files         4     4           
  Lines       346   346           
  Branches     85    85           
==================================
  Hits         68    68           
  Misses      278   278           

Flag	Coverage Δ
unittests	19% <ø> (ø)

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