Nothing in the front end should really know anything about what kind of
renderer implementation we're inspecting and indeed it's already not always
a "Fiber" in the legacy renderer and won't be for Server Components.

We typically refer to this as a "Component Instance" but the front end
currently refers to it as an Element as it historically grew from the
DOM Elements tab UI.

Except where it's actually expecting a Fiber specifically.

These are at the same level as backend/legacy.

This clarifies that anything outside of this folder shouldn't ideally refer
to a "Fiber".

Clears the root folder of the backend a bit to clarify which parts are entry points.

This follows from these same files being in packages/shared/ since they're
shared by multiple renderers.

These are not used outside the renderer itself and shouldn't communicate
via Fibers externally.

console.js calls getStackByFiberInDevAndProd separately but that's
a separate source of abstraction violations.