`.mocharc.js` and `.prettierrc.js` are tracked for code coverage

[JustinBeckwith]

Since moving to *.js files for both of these, they're suddenly getting picked up by c8 and bringing our code coverage numbers down :) Would be lovely to ship an updated c8 config!

[bcoe]

@JustinBeckwith before we ship an updated c8 config, I think there's a strong argument for this to just be ignored by default in c8's ignore rules.

[bcoe]

see: istanbuljs/schema#14 ... but, refreshing myself with our approach we actually still need to add these value to our own config.

[danielbankhead]

Idea: we can remove these files altogether if we want by moving the config to package.json (bonus: makes the base directory smaller):

Ex: https://github.com/danielbankhead/picture-playlist/blob/main/package.json