Upgrade prettier to v2

[ahutchings]

The previous glob configured in the lint:prettier task (src/**/*) did not match the implementation files directly under src/, only files 1 level down from that (in src/__tests__/), so I upgraded prettier to v2 in order to take advantage of the new prettier --write . support.

• Changed to a blacklist strategy (using .prettierignore) rather than whitelist so that editors configured to run prettier on save only run on files we care about.
• Ran prettier on all files via yarn lint:prettier
• Dropped Node.js 8 from the test matrix, since it is end-of-life and no longer supported by Prettier in v2