feat: check in a cspell config file

[bsmth]

Description

This PR gets a basic cspell config checked in that we can start using in our IDEs and on the CLI. This will not cover everything and allow us to spellcheck the entire repo from the beginning, but we can improve / use it as we go along.

Motivation

Adding en-US as preferred dictionary. Incrementally adding project words that may open the door to CI in the long term.

Related issues and pull requests

#12522

CC @nschonni

[dipikabh]

Making a note that @bsmth has also opened a discussion around this topic: https://github.com/orgs/mdn/discussions/437

I am okay with this PR with the context and caveats recorded in the discussion. Let's land this after a no-objection in Monday's Editorial.

[nschonni]

You may want to skip adding in the NPM dependency and script target, but you may want to add the suggested extension in https://github.com/mdn/content/blob/main/.vscode/extensions.json as part of t his first pass

[dipikabh]

Thanks for your work on this, @bsmth 👍. Let's land this and see how it rolls.
(Note: there's no comment in the discussion or from Editorial at this point)