[v1][breaking] Make slog.Handler optional in public constructors

[robbyt]

Summary

Every public constructor in polyscript.go and the per-engine new.go files takes slog.Handler as a required positional argument. Most call sites pass slog.Default().Handler().

The compilers already default to a stderr handler in applyDefaults():

• engines/risor/compiler/options.go:91
• engines/extism/compiler/options.go:107

Top-level constructors should follow the same pattern.

Proposal

Treat a nil slog.Handler as "use the default". A library probably shouldn't write to stderr by default — a discard handler is more polite — but at minimum the constructors shouldn't require the argument.

Files

• polyscript.go
• engines/risor/new.go
• engines/starlark/new.go
• engines/extism/new.go
• engines/new.go
• (Reconsider stderr default in compilers' applyDefaults.)

This is a breaking-change candidate for v1; it removes a positional argument from the public constructor surface.