These changes originally from #6652, but pulled out to be merged
separately.
`outputBuffer` is a simpler way of asserting tests against the CLI
output instead of working directly withe Jest mock function.
`output.isTTY` is also now mutable, so that we can write tests for both
cases when the output is different based on TTY-ness (for example,
see the updated `vc whoami` tests in this PR).
* Rewrites the CLI unit tests to be TypeScript and use Jest (consistent with the unit tests in the other packages in this repo).
* The file structure of the new test files mirrors that of the `src` directory, so that it's easy to find the relevant tests.
* Code coverage is also properly set up through Jest - you can already see a big increase in coverage from this PR.
* Adds a mock API server framework, with the intention of making it possible to write granular tests of the CLI commands. Using mocks also has the benefit of not requiring `VERCEL_TOKEN` env var to be set, so 3rd party PRs will be able to have their tests run. Ultimately this will also help with test coverage, since we will be writing unit tests that test the commands' code directly.
* Converts `Output` into a proper class (which is subclassed for the mocks).
