internal/weak: add package implementing weak pointers

internal/weak: add package implementing weak pointers

This change adds the internal/weak package, which exposes GC-supported
weak pointers to the standard library. This is for the upcoming weak
package, but may be useful for other future constructs.

For #62483.

Change-Id: I4aa8fa9400110ad5ea022a43c094051699ccab9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/576297
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>