Add support for `--watch` to `forge coverage`

[0xGuybrush]

Component

Forge

Describe the feature you would like

Allow forge coverage to be ran with the watch flag, in the same way as forge test

Additional context

When trying to increase test coverage (especially across branching logic), currently I'm running forge coverage --report lcov (and then letting Coverage Gutters in VSCode) show where I still have gaps.

Since Coverage Gutters automatically picks up changes to lcov.info, having the coverage command support watch would close that loop & make the testing cycle even faster.