- Restructure the runtime function implementation.
- Rename osr_loop_nesting_level to osr_urgency and add helpers.

The motivation for the latter: I've always struggled with the
`osr_loop_nesting_level` term; it neither matches terminology of
what it's compared against (= the loop depth), nor implies what it's
used for (= osr is triggered when `loop depth < osr nesting level`).

In this CL it's renamed to `osr_urgency` to reflect that as urgency
rises, we consider more and more loops as OSR candidates.

Bug: v8:12161
Change-Id: I194ec5a3f1f02526641af1c7796ee0956b6fd3a1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3528735Reviewed-by: Patrick Thier <pthier@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79509}