Adds tag.js to published files for remark-mdx.

[manovotny]

The tag.js file is missing when you try to use the published package. 😢

Error: Cannot find module './tag'
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:581:15)
    at Function.Module._load (internal/modules/cjs/loader.js:507:25)
    at Module.require (internal/modules/cjs/loader.js:637:17)
    at require (internal/modules/cjs/helpers.js:22:18)
    at Object.<anonymous> (/Users/manovotny/Developer/remark-mdx-metadata/node_modules/remark-mdx/index.js:2:15)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
error Command failed with exit code 1.

This PR fixes that.

Tested by running yarn pack in the /packages/remark-mdx folder.

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[silvenon]

Thank you! 👍