expose ESM browser build - fixes #2436

[Rich-Harris]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Resolves #2436

Description

(No test because I'm not sure it applies here)

This adds a JavaScript module version of the browser build, for use in https://rollupjs.org/repl once rollup/rollupjs.org#153 is merged.

It did occur to me that maybe we should be minifying the browser build, since it's not exactly tiny (854kb, 154kb zipped) — thoughts?

[lukastaegert]

Very nice! There were two questions raised, though.

1. Naming:
   I would actually go with the .mjs extension (and align the extension of the main bundle in the 1.0 PR; I can take care of that). The browser does not care for extensions anyway but other bundlers like Webpack can and will use this for internal optimization.
2. Minification:
   Definitely for the browser builds to optimize for the unpkg.com scenario. We still have sourcemaps anyway. The node build, on the other hand, might profit more from native readability and editability.

[lukastaegert]

@Rich-Harris I added minification to the browser build and changed the extension to .mjs, let me know what you think.

[Rich-Harris]

Looks good — I made a small tweak (using module: true to get better minification). If we're embracing mjs and we wanted to be consistent we could maybe change dist/rollup.es.js to dist/rollup.mjs but it's probably not worth the possibility of breakage

[lukastaegert]

👍