ESM build and refactoring

[edemaine]

• Add support for ESM .coffee files in coffee-esm.mjs (autodetection)
• Build new ESM version dist/main.mjs, which Vite seems to need in New Playground tag in docs #125 (I've tested this in code I haven't yet pushed). (If I just told esbuild to convert the CJS code to ESM, you wouldn't be able to do import {compile} from '@danielx/civet'; only the default export gets preserved.)
• CoffeeScript code refactored to use ESM (but some things like Hera parser are still CJS)
• Note that default exports no longer have everything; need to import *

[edemaine]

Oh, in package.json, I'm not sure whether "exports"/"." should point to the .js or .mjs file... Vite was unhappy with the former, but it was fine when I explicitly imported ".../main.mjs".

Perhaps we should we be using "import" and "require" conditions?

[STRd6]

I think using import and require conditions is a good idea. My main goal is to have Civet "just work" whatever type of project people npm install into. And that probably means adding both ESM and CJS builds to the default dist with the proper package.json config.