chore(manifest): add keywords to package.json

[bnb]

adds the keywords property to package.json with relevant keywords, where it was previously omitted.

noticed there weren't any keywords in the npm page for the package, figured it'd only take me a minute to add some relevant ones :)

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[asyncliz]

Thank you so much for this PR! I'm sorry we didn't get around to reviewing it. I'm adding keywords in https://github.com/material-components/material-web/pull/4764 since it's faster than importing a contribution PR.

Hope we can get more PRs from you in the future!