[WIP]: chore(*): publish es versions of browser packages

[DSchau]

Description

In preparation for modern builds, we need to also publish some of our dependencies as es. We use microbundle to do that.

Currently blocked by microbundle using buble, which isn't able to minify everything as we'd want (e.g. class is still transpiled to function). This is supposed to be merged in next week or so upstream--so we'll wait for that.

Preliminary findings are that this shaves off a few kb (non-gzipped!) compared to the CommonJS transpiled version.

Note: We'll want to publish babel-preset-gatsby in a separate PR before this change.

Related Issues

[DSchau]

@wardpeet this is probably good to re-visit now that your microbundle PR was merged, right? Or at least I thought it was merged 🙃

[wardpeet]

Definitely! Modern builds PR #14289 has some babel-preset changes you might want to introduce here too. We should install microbundle@next which has the latest updates. With that all set we should be able to wrap this up.

[freiksenet]

Is anything else needs to be done here @DSchau?

[DSchau]

@freiksenet I'll need to pull it down again to check if microbundle has been updated.

I believe @wardpeet was making some upstream changes to enable Babel (instead of Buble) which will unblock us and we can ship this!

[DSchau]

OK, mostly working. Few notes:

• Need to solve withIEPolyfill transpilation in gatsby-image (microbundle —entry fails with Error: You must set "output.dir" instead of "output.file" when generating multiple chunks as if it’s trying to parse the dynamic import statement or something; we just want it ignored
• Still (weirdly) transpiling classes, so we’ll want to ensure the esmodules target makes sense

Some preliminary numbers (note: primarily these are because class and other features are still being transpiled)

Package	Format	Size
gatsby-image	cjs	21kb
gatsby-image	mjs	19kb
gatsby-link	cjs	7.8Kb
gatsby-link	mjs	7.1Kb

[DSchau]

Let's close this -- when we're closer to implementing modern bundles, this will serve as a great reference implementation of one piece that will need to be done to maximize the benefit to end users.