* 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)
* test_fmt moves out of fmt crate
* test_parse _mostly_ moves out of parse crate and into `test_snapshots.rs` (some simple tests remain)
* now there's only two fuzz targets, fuzz_expr and fuzz_module, that cover both parsing and formatting
* added a system to auto-add new snapshot entries for new test files
* took some commented-out tests in `test_parse` and converted them to snapshot tests
* moved test_fmt's verification of formatting consistency into test_snapshots
* fixed a huge derp on my part where the fmt fuzzer in #4758 was completely useless (broken by refactoring just prior to submitting the PR)
* fixed a formatting bug found by fuzzing (bound_variable.expr.roc) - that I missed earlier due to ^^^ that derp
* no longer have roc_test_utils as a dependency in fmt - which was causing problems for the wasm build
