Internal: Fix broken master due to prettier error on CHANGELOG.md

[christianvuerings]

Summary

What changed?

1. Run prettier on CHANGELOG.md file
2. Prevent future breakages by scripting the prettier run

Why?

Master broke due to an extra space in the PR title in #1890

$ prettier --check .
Checking formatting...
[warn] CHANGELOG.md
[warn] Code style issues found in the above file(s). Forgot to run Prettier?
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
error Command failed with exit code 1.
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
Error: Process completed with exit code 1.

https://github.com/pinterest/gestalt/runs/5011272978?check_suite_focus=true

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