Include some useful "stdlib" modules by default

[negz]

What problem are you facing?

Starlark doesn't have a stdlib to speak of. Functions currently only get access to these modules:

• https://github.com/google/starlark-go/tree/master/lib/proto
• https://github.com/google/starlark-go/tree/master/lib/json

How could this Function help solve your problem?

There's a few third party packages attempting to be stdlib-like for Starlark (in Go), including:

• https://github.com/qri-io/starlib (HTTP, regex, hashes, etc)
• https://github.com/tidbyt/pixlet/tree/main/runtime/modules

We should probably expose the math and time packages from https://github.com/google/starlark-go/tree/master/lib, too.