debugger: Upgrade Starlark to remove getPredeclared hack (#143)

debugger: create package

Add a simple interactive Starlark debugger. This is a copy of a debugger that has been in use at Stripe. It can be used in any starlark-go application, not necessarily one that uses skycfg.

The debugger exposes a single `breakpoint()` function to ordinary Starlark code, which starts up an interactive REPL-style debugging session, in which users can examine the Starlark thread stack and manipulate values. An example integration is in `debugger/example_test.go`, with the transcript of a sample debugging session located in `debugger/README.md`.

Stepping through code is not supported due to restrictions in starlark-go. See google/starlark-go#304 for details.

go/protomodule: add go1.23 iter support (#140)

Add `(*protoMap).Entries` and `(*protoList).Elements`, which return `iter.Seq2` and `iter.Seq` respectively, to mirror google/starlark-go#551.

Add flatten on eval (#104)

* add flatten

* update protobuf for zlib + comment

* fix test

* add flatten as an option

* remove conditional

Use in-container workspace path for GitHub Action cache.