Bake in default template to improve PackageCompiler support

[nhz2]

This fixes #15 and is an alternative to #49.

The default template "no-slides.pptx" is less than 35 kB. This PR reads it into a constant at precompile time.
This PR allows passing a Vector{UInt8} of pre-read template data in the template_path argument to write.

The main advantage of this over #49 is that it is easier to edit the default template and there is no need to find someplace to store the artifact.

I also added a .gitattributes file to prevent GitHub Windows CI from changing the file endings: https://code.visualstudio.com/docs/devcontainers/tips-and-tricks#_resolving-git-line-ending-issues-in-containers-resulting-in-many-modified-files This is to ensure the baked in data is the same bytes on all systems.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.85%. Comparing base (09e8a06) to head (ec90d2c).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   96.00%   95.85%   -0.16%     
==========================================
  Files           9       10       +1     
  Lines         476      507      +31     
==========================================
+ Hits          457      486      +29     
- Misses         19       21       +2     

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