feat(tvix/eval): emit warnings when globals are being shadowed

Change-Id: I7dae6978c2a4548382d7fa059b20ccdf35d2cf7f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6257
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/warnings.rs

@@ -6,6 +6,7 @@ pub enum WarningKind {
     DeprecatedLiteralURL,
     UselessInherit,
     UnusedBinding,
+    ShadowedGlobal(&'static str),
 }
 
 #[derive(Debug)]