Make core load any pattern engines more intelligently

[bmuenzenmeyer]

Part of #603

I am going to take this issue, as I believe it also encompasses fixing the merge conflict inside #608

• Merge Change var to const or let #608
• Defer load of engines inside module scope until absolutely necessary within build
• Fix unit tests
• Pass or alter linting
• Confirm watches still work

[bmuenzenmeyer]

Descoped #617 from this effort, citing timing.

[stale]

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