Make eslint and prettier work again

[mirka]

Proposed changes

• Install the missing eslint-plugin-eslint-comments plugin to make eslint run again.
• Add ignore patterns to eslintrc.
• Add ignore patterns to prettierignore.
• Add yarn commands for eslint and prettier (following wp-scripts conventions).

To test

yarn lint:js --fix && yarn format should fix a bunch of issues, but only in directories we want.

[mirka]

@johngodley As for commiting a batch fix of the existing lint/format errors, let's wait for a time that's convenient so we don't have to reconcile our PRs and whatnot.