Allow more ergonomic require of cjs esbuild plugin

[STRd6]

This lets us use civetPlugin = require "@danielx/civet/esbuild-plugin" instead of needing to use .default.

We can pick it up in esbuild.civet after publishing.

[edemaine]

According to the tests, this seems to break import esbuildPlugin from "../../source/esbuild-plugin.civet"... ☹️
Oh, but maybe that should be importing from dist.

[edemaine]

I read evanw/esbuild#1719 which talks about the fun history here, and followed some of the back links. It seems like conventional-changelog/commitlint#3614 might work for our context, though I don't quite understand why. 🙂

[STRd6]

This weird hybrid export hack works but it does create a warning from esbuild.

The only reason for the export default is so ts-node can do an esm import for test coverage purposes.

🤷