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/language_server/src/analysis/analysed_doc.rs

@@ -1,4 +1,5 @@
 use log::{debug, info};
+use roc_fmt::annotation::MigrationFlags;
 use std::collections::HashMap;
 
 use bumpalo::Bump;
@@ -92,7 +93,8 @@ impl DocInfo {
         let arena = &Bump::new();
 
         let ast = Ast::parse(arena, source).ok()?;
-        let fmt = ast.fmt();
+        let flags = MigrationFlags::new(false);
+        let fmt = ast.fmt(&flags);
 
         if source == fmt.as_str() {
             None

crates/language_server/src/analysis/parse_ast.rs

@@ -1,5 +1,5 @@
 use bumpalo::Bump;
-use roc_fmt::Buf;
+use roc_fmt::{annotation::MigrationFlags, Buf};
 use roc_parse::{
     ast::{Defs, Header, SpacesBefore},
     header::parse_module_defs,
@@ -40,12 +40,12 @@ impl<'a> Ast<'a> {
         })
     }
 
-    pub fn fmt(&self) -> FormattedAst<'a> {
+    pub fn fmt(&self, flags: &MigrationFlags) -> FormattedAst<'a> {
         let mut buf = Buf::new_in(self.arena);
 
-        roc_fmt::header::fmt_header(&mut buf, &self.module);
+        roc_fmt::header::fmt_header(&mut buf, &self.module, flags);
 
-        roc_fmt::def::fmt_defs(&mut buf, &self.defs, 0);
+        roc_fmt::def::fmt_defs(&mut buf, &self.defs, flags, 0);
 
         buf.fmt_end_of_file();