Update AVA and enable testing on Node 14

[fregante]

Based on #67

Blocked by avajs/babel#13 and especially babel/babel#6737

This PR won't be merged. I think the only way to support this mess is to pre-bundle the test files like I did in Refined GitHub: refined-github/refined-github#3206

The problematic combination is:

• JSX in test files
• type: module in main module

Babel is needed to support JSX in test files, but there's no way to then import ./index.js since it's a type: module that can't be require'd

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Note: The tests on master currently already work with AVA@3 as soon as I drop type: module.

[fregante]

Possible alternative solution: look into Jest

[fregante]

I resolved by using Rollup (which was already there) to create a CJS bundle and test against that in a folder marked as type: "commonjs"