Support for emitting a standalone metafile

There are some uses cases where I want the metafile, for example to pipe it through the bundle analyzer, but where I don't actually care about bundled files.

The problem is that the `--metafile` flag requires the `--outdir` option to be set. IMO that's an unnecessary limitation that complicates the process of getting a metafile.

I think not only `--metafile=foo.json` should just work, but `--metafile` also should work, and just write the metafile to stdout.

Start by tracing the CLI handling for --metafile and its dependency on --outdir. Check how metafile output is currently written, then verify that a standalone path works without bundled files and that bare --metafile writes to stdout; use the existing CLI test workflow if available.

Written by the indexing model from the issue text.