Add support for default values in configuration variables

[fvictorio]

Describe the issue

Hardhat 2's configuration variables let you do this:

export const ethMainnetUrl = vars.get(
  "ETH_MAINNET_URL",
  "https://rpc.ankr.com/eth",
);

As far as I know, there is no equivalent functionality in Hardhat 3, nor a way to work around it.

This is a migration blocker for createx.