[scopes] Resolve original scope chains using source map 'scopes' info

The algorithm to obtain the original scope chain is straight-forward:

  1) Find the inner-most generated range that contains the
     CallFrame's paused position.

  2) Does the found range have an associated original scope?

     2a) If no, return null. This is a "hidden" range and technically
         we shouldn't be pausing here in the first place.
         This code doesn't correspond to anything in the authored code.

     2b) If yes, the associated original scope is the inner-most
         original scope in the resulting scope chain.

  3) Walk the parent chain of the found original scope outwards. This is
     our scope view. For each original scope we also try to find a
     corresponding generated range that contains the CallFrame's
     paused position. We need the generated range to resolve variable
     values.

The CL also adds a couple of unit tests to document the existing
behavior and design choices. These might need to be revisited
depending on how source map generators actually choose to emit
scopes.

R=pfaffe@chromium.org

Bug: 40277685
Change-Id: Ia033d3a5f8d1c60be5059c05da770e220940538e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/5833561
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Philip Pfaffe <pfaffe@chromium.org>