* Unify parsing of string literals and scalar literals, to (e.g.) ensure escapes are handled uniformly. Notably, this makes unicode escapes valid in scalar literals.
* Add a variety of custom error messages about specific failure cases of parsing string/scalar literals. For example, if we're expecting a string (e.g. a package name in the header) and the user tried using single quotes, give a clear message about that.
* Fix formatting of unicode escapes (they previously used {}, now correctly use () to match roc strings)
As previously discovered with #4464, it's easy to accidentally mis-use the State value returned on the Err path.
There were mixed assumptions about what that State represents: (1) the State where the error occurred, or (2) the State at the beginning of the thing we were just parsing.
I fixed this up to always mean (2) - at which point we don't actually need to return the State at all - so it's impossible for further discrepency to creep in.
I also took the liberty to refactor a few more methods to be purely combinator-based, rather than calling `parse` directly.
This removes the need to explicitly pass thru min_indent when using the parser combinators.
My ultimate goal here is to evolve the current parser closer toward a purely combinator-based parser,
at which point we can more easily transition smoothly to a formal(ish) grammar, or expand the meanings of combinators
to include things like:
* Incremental (re)parsing
* Unified parsing and formatting code
* Better error recovery
* Using the main parser directly for syntax highlighting
