webium: Separate TrackedElementHandler from HelpBubbleHandler in mojo

This change refactors WebUI help bubble's element tracking functionality
into a new TrackedElementHandler mojo interface in a new file at
//ui/webui/resources/js/tracked_elements/tracked_element.mojom. The file
is imported to the existing help_bubble.mojom.

This improves modularity and allows a WebUI to track element bounds
without depending on Help Bubble. The bounds will be used in Webium for
anchoring secondary UIs onto the top-chrome WebUI.

Both the TS and C++ code of this interface are still in Help Bubble. In
follow up CLs they will be refactored out and moved into tracked
element's owned folder.

This CL does not add any observable behavioral changes.

Bug: 40243115, 393144165
Change-Id: I23d1ac3b8bf82cc32d118402884f481bc3fec177
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6840786
Reviewed-by: Dana Fried <dfried@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Cr-Commit-Position: refs/heads/main@{#1501559}