Install Prettier via package.json

[benpickles]

This adds a package.json with Prettier and means that developers/CI will use the same version. It also ensures that NPM packages are cached on GitHub Actions which fixes #1169.

[codecov]

Codecov Report

Merging #1170 (a5761cc) into main (1e8fa3a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1170   +/-   ##
=======================================
  Coverage   91.55%   91.55%           
=======================================
  Files          32       32           
  Lines        3932     3932           
=======================================
  Hits         3600     3600           
  Misses        332      332           

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[benpickles]

I was going to re-add the filter to only run Prettier when there have been changes in datasette/static but that would mean it wouldn't run when the package is updated. That plus the fact that the last run of the job took only 8 seconds is why I decided not to re-add the filter.

[simonw]

Thank you very much!