Allow piping data into the CLI

[RobinMalfait]

This PR will allow you to pipe data to the cli (and the standalone binaries)

We use - as a placeholder for the input to indicate that we want to read from stdin. There is no way to tell if you getting data via stdin or not (unless you actually get the data). We could use !process.stdin.isTTY, the issue is that this is also false in CI-like environments.

If we actually receive data, then everything works fine as expected, however if we don't receive data then we wait forever. This makes sense if you think about this example:

super-long-running-process-that-gives-you-data-after-10-minutes | npx tailwindcss -o output.css

When do we know that we have to stop watching? We could wait for 100ms, 1s, ... but we don't know when to stop. Halting problem!

As an alternative, a lot of tools use the - instead of the input file to indicate that we want to wait for stdin. You can try it yourself: cat - it will just hang forever.

Long story short, this now works:

cat input.css | npx tailwindcss -i - -o output.css

Fixes #6865