Add solidity-coverage support

[kanej]

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Investigate the vscode apis for displaying line by line coverage inline and use them to support solidity-coverage.

Solidity coverage is a hardhat extension. We should determine from config whether it is enabled for a project.

I am unsure when coverage calculation should be generated. Perhaps we monitor for changes in the output file? Or add a run coverage task if it is detected in hh config and then process its output.

[kanej]

An example of and extension that provides coverage support inline is: https://marketplace.visualstudio.com/items?itemName=ryanluker.vscode-coverage-gutters