Update linting setup, fix

Correct missing dep

Don't replace `null` in RFC content

Add util to list frontmatter from all

Add lint rules to ensure every key under `prs` is stage and each key tha

t is there is a valid URL for a PR on emberjs/rfcs

Add script to update prs in frontmatter

allow release-date and release-versions to be null

update code-comment to be less confusing