Make htmx a peer dependency of each extension

[scrhartley]

Description

PR #123 made htmx a dependency of each extension instead of a dev dependency.
One reason given was to ensure a minimum version of htmx is installed that's compatible with the extension.
Unfortunately this doesn't work in practice, so rather than flagging any version mismatch, if there's a conflict NPM will instead create its own nested version of htmx for the extension and that will end up in the bundle as well.
Changing the dependency to a peer one solved the problem and extensions/plugins are the intended use case for peer dependencies.

With my own test specifying a v1 release for htmx with a v2 of the SSE extension,
both a v1 and v2 release of htmx ended up in the final bundle, increasing bundle size from 65K to 115K.

Htmx version: 1.9.2 / 2.0.6
Used extension(s) version(s): 2.2.3 of SSE, plus a WIP version of my own extension

Testing

I tested with my own extension to verify the behavior of normal vs. peer dependencies.
I ran npm install and npm run test for each extension.

Checklist

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

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