feat(tvix/eval): allow ignoring locals by prefixing with _

This is a common idiom in both Nix and other languages when a local is
declared without actually being used.

Since Tvix warns for unused locals, having this available is useful
and can be included in the final error message as a suggestion if an
unused variable is intentional.

Change-Id: Ia85f704ba183499a3bae657c58166e2e29f9bde5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6320
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/mod.rs

@@ -884,10 +884,11 @@ impl Compiler {
             if let Some(Local {
                 node: Some(node),
                 used,
+                name,
                 ..
             }) = self.scope_mut().locals.pop()
             {
-                if !used {
+                if !used && !name.starts_with('_') {
                     self.emit_warning(node, WarningKind::UnusedBinding);
                 }
             }