Parsing support for snake_case identifiers

In this initial commit, I have done the following:

- Added unit tests to roc_parse's ident.rs file to cover at least the
  simplest Ident enum cases (Tag, OpaqueRef, and simple Access)
- Added '_' as a valid "rest" character in both uppercase and lowercase
  identifier parts
- Updated the test_syntax snapshots appropriately

There is still a lot left to do here. Such as:

- Do we want to allow multiple '_'s to parse successfully?
- Handle qualified access
- Handle accessor functions
- Handle record update functions
- Remove the UnderscoreInMiddle case from BadIdent
- Write unit tests for Malformed Idents

I am not a "Rustacean" by any means, but have been through the Book in
years past.  Any feedback on the way I wrote the tests or any other part
of the implementation would be very appreciated.

crates/compiler/problem/src/can.rs

@@ -763,8 +763,8 @@ impl RuntimeError {
                 record: _,
                 field: region,
             }
-            | RuntimeError::ReadIngestedFileError { region, .. } => *region,
-            RuntimeError::InvalidUnicodeCodePt(region) => *region,
+            | RuntimeError::ReadIngestedFileError { region, .. }
+            | RuntimeError::InvalidUnicodeCodePt(region) => *region,
             RuntimeError::UnresolvedTypeVar | RuntimeError::ErroneousType => Region::zero(),
             RuntimeError::LookupNotInScope { loc_name, .. } => loc_name.region,
             RuntimeError::OpaqueNotDefined { usage, .. } => usage.region,