Add --coverage option to build scripts for tests

[rbuckton]

This adds a --coverage option to our build scripts for use with runtests, runtests-parallel, and runtests-watch. When --coverage is specified, tests will be run through c8 and dump an lcov format file to coverage/lcov.info for use with a code coverage extension for VS Code or other tooling.

[rbuckton]

c8 isn't strictly necessary. We could do much the same thing by just specifying the NODE_V8_COVERAGE environment variable, but it's helpful to have it produce a formatted coverage file rather than requiring a second step.

[jakebailey]

I've been wanting this officially for a while, thanks!

(c8 hereby runtests-parallel was an ok thing up until now)

[jakebailey]

Can we dev dep on c8 and run it directly instead of using npm exec?

[rbuckton]

It's easier to continue using npm exec to avoid having to deal with trying to spawn a local package bin and deal with multiple platforms myself, though I can add --prefer-offline or --offline to have it prefer the local version.

[rbuckton]

To clarify, I am both continuing to use npm exec for convenience and also adding c8 to devDependencies.

[jakebailey]

It's easier to continue using npm exec to avoid having to deal with trying to spawn a local package bin and deal with multiple platforms myself, though I can add --prefer-offline or --offline to have it prefer the local version.

What do you mean by platforms? It should be at node_modules/c8/bin/c8.js; we execute eslint and mocha using similar methods.