Added an underscore to allowed domain label symbols

[icehaunter]

This PR adds an underscore to allowed domain label symbols. We encountered an issue with this when some of our users complained that a google cloud storage URL wasn't auto-linked properly when it had one or more underscores.

As an example, currently https://gcs_test_env.storage.googleapis.com/file.pdf is auto-linked as https://gcs link + _test_ plaintext + env.storage.googleapis.com/file.pdf link.

Sidenote, the emojiStr change was added automatically by the prettier format-on-commit. I can remove that change if you'd like.