This was incorrectly ported from Rust to Go.
When provided, `treefmt` will take the contents of stdin and place them into the file provided with the `--stdin` flag, then format it according to the configured formatters.
If the file doesn't exist it is created. If it exists, it is first truncated and then populated with stdin.
Signed-off-by: Brian McGee <brian@bmcgee.ie>
