[Extensions] Create a `browser` namespace as an alternative to `chrome`

Before this change extensions had to use the `chrome` namespace to
access extension APIs.

After this change we introduce a new `browser` namespace (behind a
feature flag) that is available to call extension APIs wherever they are
available. The exceptions to this is WebUI.

The API objects pointed to by each namespace are the same so they can be
used interchangeably if desired.

The deprecated `app` API is never available on `browser` though.

There is no change to the `chrome` namespace.

Bug: 401226626
Change-Id: I797ff4c56022981fe75ddb9f54cf70e5a5b5c454
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6306847
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Justin Lulejian <jlulejian@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1434960}