fix(tvix/eval): add unimplemented __curPos and builtins.filterSource

This commit adds __curPos (to the global scope, yuck) and
builtins.filterSource.  These are not implemented; forcing them will
produce the same result as `throw "message"`.

Unfortunately these two post-2.3 features are used throughout
nixpkgs.  Since an unresolved indentifier is a catchable error, this
breaks the entire release eval.  With this commit, it simply causes
those broken packages that use these features to appear as they are:
broken.

Change-Id: Ib43dea571f6a9fab4d54869349f80ee4ec5424c2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10297
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Autosubmit: Adam Joseph <adam@westernsemico.com>

tvix/eval/src/errors.rs

@@ -42,6 +42,7 @@ use crate::{SourceCode, Value};
 pub enum CatchableErrorKind {
     Throw(String),
     AssertionFailed,
+    UnimplementedFeature(String),
     /// Resolving a user-supplied angle brackets path literal failed in some way.
     NixPathResolution(String),
 }