feat(instrumenter/mutators): Implement missing AssignmentOperatorMutator

[Qvdpost]

This pull request contains the missing AssignmentOperatorMutator implemented for:
'+=' = '-=',
'-=' = '+=',
'=' = '/=',
'/=' = '=',
'%=' = '*=',
'<<=' = '>>=',
'>>=' = '<<=',
'&=' = '|=',
'|=' = '&=',
'&&=' = '||=',
'||=' = '&&=',
'??=' = '&&=',

This list also contains the '??=' which is not present in the Stryker-js documentation. We are not sure how the documentation should be updated to reflect this change, given that the operator might be specific to JavaScript.
@nicojs

Breaks :
instrumenter integration
should not place disabled mutants:

  AssertionError: expected value to match snapshot instrumenter integration should not place disabled mutants 1
  + expected - actual


  at arrangeAndActAssert (test/integration/instrumenter.it.spec.ts:126:41)
  at Context.<anonymous> (test/integration/instrumenter.it.spec.ts:52:5)

[nicojs]

Great to see such a complete PR. Great job. Got some small remarks.

Fix the build:

✖ 1 problem (1 error, 0 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

This is a formatting check, you can use npm run lint:fix from the root. That should fix it.

  AssertionError: expected value to match snapshot instrumenter integration should not place disabled mutants 1
  + expected - actual


  at arrangeAndActAssert (test/integration/instrumenter.it.spec.ts:126:41)
  at Context.<anonymous> (test/integration/instrumenter.it.spec.ts:52:5)

To fix that, you can update the snapshots. Use CHAI_JEST_SNAPSHOT_UPDATE_ALL=true npm run test from the instrumenter package. That updates the snapshots. Probably this mutator added a mutation.

[Qvdpost]

Thank you for the suggestions!

We applied each of your remarks to the best of our understanding. All tests now pass (yay!).

Please let us know if there are any other issues that need be resolved.

Regards,
Lily & Quinten

[nicojs]

Oh haha. Great to see your mutation is being placed correctly 😅

[nicojs]

Looks great 👍

[nicojs]

One of the e2e tests failed, because we used a += and -= in that particular test project, which in turn changed the actual number of killed and survived mutants. I've updated it 🎉.

[LilyOSullivan]

Thank you for helping us fix it, and guiding us through the pull request! 🎉

[nicojs]

Also: 100% mutation score for this new mutator: https://dashboard.stryker-mutator.io/reports/github.com/stryker-mutator/stryker-js/master?module=instrumenter#mutant/mutators/assignment-operator-mutator.ts

[nicojs]

This feature just released in Stryker 5.5, see https://twitter.com/stryker_mutator/status/1463193759726948358