feat: add more TypeDocs; add default for Runner.create; export Gas & NEAR

[chadoh]

The underlying near-runner functions have TypeDoc, but these do not get passed through. If people use near-runner-ava, this adds friendly docs for them, too.

This also makes exports Gas and NEAR from near-units, so that people don't have to have both in their package.json or know the other package name.

And finally, it sets a default no-op function for Runner.create so it can be instantiated without an argument:

const runner = Runner.create()
runner.run(async ({ root }) => {...})
runner.run(async ({ root }) => {...})