Element.currentPatch reflects current status of patchfor operations

Added an IDL-exposed PatchStatus, which reflects the status of an
ongoing patch operation initiated by <template patchfor>.

The exposed interface exposes the detached template element,
as well as a promise that resolves when the patching is complete.

To be added: rejecting the promise on errors, allowing to abort the
patch via the PatchStatus interface.

Bug: 431374376
Change-Id: I496447296b3f49e5f197d7e4148e7e8ffda604cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6757120
Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Reviewed-by: Noam Rosenthal <nrosenthal@google.com>
Commit-Queue: Noam Rosenthal <nrosenthal@google.com>
Cr-Commit-Position: refs/heads/main@{#1489336}