Design an ESM compatible plugin system for HHv3

[kanej]

HHv3's intended switch to ESM will have knock on effects for HH's plugin system.

We need to approach plugins, a key feature of the Hardhat ecosystem, carefully. We therefore want to bring the design work forward to evaluate the implications early.

TODO

• Create a design doc explaining the problem of ESM and HH plugins #4921
• Create design proposals
• Evaluate upgrade costs for large HH projects
• Finalize design

[kanej]

We have put together a prototype of an ESM compatible plugin system that you can find under the v-next branch.