Add a linting check in CI pipeline

[david-mears-2]

npm run lint shouts if things are malformed - results in the CI 'unit test' workflow returning as failed. This is observed for commit a533c93.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.11%. Comparing base (e42ef1d) to head (71e2d13).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #18   +/-   ##
=======================================
  Coverage   98.11%   98.11%           
=======================================
  Files          25       25           
  Lines         584      584           
  Branches      144      144           
=======================================
  Hits          573      573           
  Misses         10       10           
  Partials        1        1           

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[EmmaLRussell]

Looks good. If you wanted to, could run Lint in a separate job or after the tests. I find it a bit annoying if my tests don't get run because of lint but YMMV, and it all has to pass in the end...