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/parse/src/normalize.rs

@@ -855,6 +855,7 @@ fn remove_spaces_bad_ident(ident: BadIdent) -> BadIdent {
             position: Position::zero(),
             declaration_region,
         },
+        BadIdent::TooManyUnderscores(_) => BadIdent::TooManyUnderscores(Position::zero()),
         BadIdent::QualifiedTag(_) => BadIdent::QualifiedTag(Position::zero()),
         BadIdent::WeirdAccessor(_) => BadIdent::WeirdAccessor(Position::zero()),
         BadIdent::WeirdDotAccess(_) => BadIdent::WeirdDotAccess(Position::zero()),