DebugVisualizers: Add natvis for base::span

`base::span` is a wrapper around a pointer and a size. Currently we can
only look at the first element (at the data_ pointer) and the size. This
change allows us to view every element in the span.

Change-Id: I59f848c8c12ab059de5f6639c7c7be2e008730dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6489526
Commit-Queue: Michael Tang <tangm@microsoft.com>
Commit-Queue: Jesse McKenna <jessemckenna@google.com>
Auto-Submit: Michael Tang <tangm@microsoft.com>
Reviewed-by: Jesse McKenna <jessemckenna@google.com>
Cr-Commit-Position: refs/heads/main@{#1451310}