Add .prettierignore file

[ewels]

Direct copy of .gitignore.

Hopefully can get rid of this file again one day, see prettier/prettier#4708

Fixes #1504

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[drpatelh]

💥

[codecov]

Codecov Report

Merging #1505 (aa170b6) into dev (0412603) will not change coverage.
The diff coverage is n/a.

❗ Current head aa170b6 differs from pull request most recent head d23ebd8. Consider uploading reports for the commit d23ebd8 to get more accurate results

@@           Coverage Diff           @@
##              dev    #1505   +/-   ##
=======================================
  Coverage   64.92%   64.92%           
=======================================
  Files          52       52           
  Lines        6059     6059           
=======================================
  Hits         3934     3934           
  Misses       2125     2125           

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