Move idiomorph-htmx extension over from Idiomorph repo

[botandrose]

Description

This PR moves the idiomorph-htmx extension over from the Idiomorph repo. I'm trying to slim down Idiomorph's core at the moment, and this repo seems like the correct place for this code, anyways, since htmx composes Idiomorph, and not vice-versa.

I also moved the test suite over from Idiomorph, as well, and got it passing.

Lastly, I'm not familiar with htmx, so I may have done something wrong or suboptimal here.

Open questions

Maybe you'll want to name this extension something else? It was just named "morph" in the Idiomorph src, but I changed it to "idiomorph" here for consistency.

Checklist

• [x] I have read the contribution guidelines
• [x] I ran the test suite locally (npm run test) and verified that it succeeded

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