feat(tazjin/rlox): Parse if statements

Change-Id: I2352d75a3f02d65a5a2d04fb2cc4daa50f11ca1e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2321
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/rlox/src/errors.rs

@@ -7,6 +7,7 @@ pub enum ErrorKind {
     ExpectedSemicolon,
     ExpectedClosingBrace,
     ExpectedVariableName,
+    ExpectedToken(&'static str),
     TypeError(String),
     UndefinedVariable(String),
     InternalError(String),