ember-modal-dialog 4.0.0-alpha.0, Drop support for <=Ember 3.19, Node 10

[elwayman02]

• Bumps ember-wormhole to 0.6.0. This was causing a mismatch between babel 6 & 7 in consuming apps, which created build errors.
• Drops transitive dependency on an older version of ember-cli-htmlbars
• This will require a major release, since ember-modal-dialog 4.0.0-alpha.0 has symobolically (but not technically) dropped support for <= Ember 3.19: Drop support for Ember 3.19 and below yapplabs/ember-modal-dialog#332
• Also drops Node 10 support, which was dropped by ember-modal-dialog: Add volta config yapplabs/ember-modal-dialog#333

Ran yarn test:ember, yarn test:node, and yarn test:test-apps locally to confirm no failures.

[elwayman02]

I'm not able to restart the builds. It seems they were passing but one of them encountered a hiccup installing ember-source and that caused the rest to get canceled. Pushing an empty commit to trigger the builds again.

[elwayman02]

Updated the PR title to make it more descriptive for any CHANGELOG auto-generation.

[elwayman02]

@rwwagner90 please review when you have the chance, thank you! :)