feat(cli): add read-only flag

[brandonspark]

What:

This PR adds in a --read-only flag, which causes smlfmt to skip interactive confirmation in the negative sense, only parsing all files without previewing or formatting them. It's called read-only, because it just causes the files to be parsed/read, without overwriting or formatting.

Why:

I'm interested in using smlfmt to detect syntax errors in code. I don't need the preview, since I just want the error output, so --preview-only isn't what I want.

How:

Just added a command-line flag.

Test plan:

This is what the flag does on my machine:

[shwestrick]

Seems like a nice feature!

I'm not sold on the name --unforce. The name --no-write might be more self-explanatory? Or perhaps --read-only?

Thoughts?

[brandonspark]

@shwestrick I'm fine with --read-only!

[shwestrick]

Great! Thanks for putting it together.