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/repl_eval/src/gen.rs

@@ -25,8 +25,9 @@ pub fn format_answer<'a>(arena: &'a Bump, answer: Expr<'_>) -> &'a str {
         Expr::Closure(_, _) => "<function>",
         _ => {
             let mut expr = roc_fmt::Buf::new_in(arena);
+            let flags = roc_fmt::annotation::MigrationFlags::new(false);
 
-            answer.format_with_options(&mut expr, Parens::NotNeeded, Newlines::Yes, 0);
+            answer.format_with_options(&mut expr, Parens::NotNeeded, Newlines::Yes, &flags, 0);
 
             expr.into_bump_str()
         }