feat(tazjin/rlox): Synchronise parser state after errors

This lets the parser collect multiple errors instead of returning
after the first one, with some optimistic synchronisation after
encountering something that looks wonky.

Change-Id: Ie9d0ce8de9dcc7a3d1e7aa2abe15f74cab0ab96b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2236
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/errors.rs

@@ -3,6 +3,7 @@ pub enum ErrorKind {
     UnexpectedChar(char),
     UnterminatedString,
     UnmatchedParens,
+    ExpectedExpression(String),
 }
 
 #[derive(Debug)]