reset back to using esm

[mansona]

so there was an announcement a while ago that all GitHub actions would use Node 14 but it turns out that was a bit of a misleading announcement. That only applies for the environment that the actions are run in but not the actions themselves 🤦

There is an open issue requesting that actions have the ability to run in Node 14 that seems to be blocked for some reason.

Long story short: I was a bit preemptive removing the use of esm from ember-asset-size-action since we can't make use of native esm modules just yet 😞

Edit: turns out that mocha 9 doesn't work in this setup because it is enabling partial esm support even though you need the flag for that to work correctly 🙃