Include only needed files to npm package

[rubiesonthesky]

Hi!

It seems that this package is 444 kB by npm's report.

I think there are a lot unnecessary files included in the published package: https://www.npmjs.com/package/@lezer/markdown?activeTab=code

For example

• .mocharc.cjs
• CHANGELOG.md (could be included, but could be just linked to repo)
• build.js
• rollup.config.js
• tsconfig.json
• test dir
• bin dir only includes build-readme script?
• src/README.md seems same as ./README.md?

So only files that should be included in ´files` section in package.json seem to be

• dist
• README.md
• package.json

There may be other files to be included depending on your preferences. But I think usually only these files are needed :) That probably about halves the package size.