CI: enable Prettier checking

[chalin]

• Enables Prettier checking over all non-ignored resources to CI by creating NPM scripts that contributors can run too
• Closes Introduce prettier workflow #2179 by superseding it
• Closes CI: check file formatting, including markdown #1063, though we'll have to progressively relax the .prettierignore rules

✅ Note that this PR was seeded with a formatting issue in the README.md file. The build did fail (for details, see the comment below). If the build fails, then that will confirm that the changes in this PR are working. I'll then remove the README.md file change.

[chalin]

Hurrah, the build failed as expected (https://app.netlify.com/sites/opentelemetry/deploys/63e27aaf3687ae00080394da):

11:22:44 AM: [warn] README.md
11:22:44 AM: [warn] Code style issues found in the above file. Forgot to run Prettier?
11:22:44 AM: ERROR: "check:formatting" exited with 1.
11:22:44 AM: ​
11:22:44 AM:   "build.command" failed                                        

So I'll remove the seeded error.

[chalin]

Build is green. This is ready for review.