Add files to package.json

[styfle]

Description

This will prevent publishing dev files to npm so that marked users who run npm install marked will not need to download any unnecessary files.

This config will replace the .npmignore file which is why I deleted it.

Contributor

• no new tests required for this PR.

Committer

In most cases, this should be a different person than the contributor.

• Draft GitHub release notes have been updated.
• CI is green (no forced merge required).
• Merge PR

[UziTech]

I like it. That way we won't have to remember to add to .npmignore if we add other config files

[styfle]

/cc @davisjam @joshbruce