Enforce indents/continuations must follow a line break

Currently, it's possible get `FormattingData` into a semi-invalid state pretty easily.
https://github.com/integrated-application-development/pasfmt/blob/bd8e1e874b3dbf19eeece0c8c2385776294dfd5b/core/src/lang.rs#L373-L379

You can set `indentations_before` to a non-zero value despite `newlines_before` being zero.
There's no good reason to want indentations before a token without any line breaks preceding them. It's not even useful for alignment, because you would want to use spaces for that.
The same is true of continuations.

It should be possible to leverage the type system to enforce that every indentation or continuation is preceded by a line break. In particular the `NonZero*` types in [std::num](https://doc.rust-lang.org/nightly/std/num/index.html) would be useful.