DOM: Implement the `last()` Observable operator

See https://github.com/WICG/observable/pull/133

. This CL implements the
`last()` operator which returns a Promise that resolves to the last
item that the source Observable emits, and rejects with a RangeError
if the Observable completes prematurely.

For WPTs:
Co-authored-by:  <ben@benlesh.com>

R=dizhangg@chromium.org

Bug: 40282760
Change-Id: Id1ec2af152c288ae81dfe383dfa9a7f166cfb375
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5417373


Reviewed-by: Di Zhang <dizhangg@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282037}