apply Prettier formatting to lib

[brodybits]

• move extends: plugin:prettier/recommended to top-level .eslintrc.yml
• apply Prettier formatting to lib

This is basically a continuation of the Prettier formatting that was already applied to test in merged PR #123.

Note that the rationale behind the Prettier options used is given in the updated description of merge PR #123.

• I would rather have further improvement of test coverage with the testing strategy discussed in Improve test coverage with future-proof testing strategy #72 before merging this proposal. While it should be safe to let Prettier reformat the code, I think it would be better practice to further improve the Stryker mutation testing score before updating such a large amount of the code.

[karfau]

this one has tons of conflicts so I started from scratch in #447 which also includes the new example code.