Implement treefmt's (nascent) Stdin Spec

[jfly]

treefmt will be adding a Stdin Spec. See numtide/treefmt#586 for details.

We should implement it, which means deprecating and removing the current behavior:

1. Read from stdin if no positional arguments are passed.
2. Use the filename specified via --filename: Add --filename flag for overriding the filename when using stdioTarget #264

The new behavior will (likely, wait for treefmt's spec to be approved) be to:

1. Only read from stdin if the --stdin option is passed.
2. The user must specify a filename as a positional argument.

A benefit of deprecating the implicit read from stdin behavior is users won't get confused by nixfmt "freezing", as reported in #301.