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/can/src/pattern.rs

@@ -273,6 +273,14 @@ pub fn canonicalize_def_header_pattern<'a>(
                 region,
             ) {
                 Ok((symbol, shadowing_ability_member)) => {
+                    if name.contains("__") {
+                        env.problem(Problem::RuntimeError(RuntimeError::MalformedPattern(
+                            MalformedPatternProblem::BadIdent(
+                                roc_parse::ident::BadIdent::TooManyUnderscores(region.start()),
+                            ),
+                            region,
+                        )));
+                    }
                     let can_pattern = match shadowing_ability_member {
                         // A fresh identifier.
                         None => {