feat(tvix/eval): implement ? operator (single-level only)

This makes it possible to check things like `{} ? a` with a single
level of nesting.

Change-Id: I567c36fcfd2f9e2f60071acd3ebfe56dea59b26f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6161
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/opcode.rs

@@ -47,6 +47,7 @@ pub enum OpCode {
     OpAttrPath(usize),
     OpAttrsUpdate,
     OpAttrsSelect,
+    OpAttrsIsSet,
 
     // Lists
     OpList(usize),