Create a public function for source formatting

[cocuh]

Alternative solution for #652.

In most cases, users don't need FormattingError enum itself, and just serializing the error message makes things easier in later code chagnes I believe.

[udoprog]

You don't have to use anyhow to hide the error enum. A public struct with a private field implementing std::error::Error would achieve the same result without the feature test.

[udoprog]

Woops, the auto merged triggered prior to building 🤔. No big deal, I'll fix it when I get around to it.

Thanks!

[cocuh]

Ooop, it seems my commit makes build failure. Let me create PR for the fix.