Use feature resolver version 2

This resolves the warning cargo has been emitting for a while regarding
having to fall back to the old feature resolver version.

Using the new resolver version doesn't seem to affect us, but it's
better to be with the new version since it's the default for edition
2021 Rust (but not for virtual workspaces for some reason).

See
- https://doc.rust-lang.org/cargo/reference/resolver.html#feature-resolver-version-2
- rust-lang/cargo#10112

Author: zaneduffield

Cargo.toml

@@ -6,6 +6,8 @@ members = [
     "front-end",
 ]
 
+resolver = "2"
+
 [profile.release]
 lto = true
 codegen-units = 1