Coverage script combining results from hardhat and foundry tests

[miguelmtzinf]

#322 added a script that runs both test suites and combines results into one single lcov file. However, this is no longer working due to the merge of #382, which is removing outdated logic for hardhat-foundry compatibility (e.g. preprocessor) and moving from npm dependencies to gitmodules/foundry ones.

A coverage script that includes results from both test suites would give a more complete vision of code coverage of the repo. For now, the repo contains coverage scripts for Hardhat and Foundry, separately.

NomicFoundation/hardhat#3961
foundry-rs/foundry#6609