[flow analysis] Align _attachFinally with the specification.

This change refactors the `_attachFinally` method in `_FlowAnalysisImpl`
to better align with the flow analysis specification.

Notable changes include:

- Moving `attachFinally` to the `FlowAnalysisImpl` class rather than
  implicitly passing the `afterTry` model as `this`. This makes the
  arguments to `attachFinally` match those in the spec.

- Extracting the handling of a single variable to a separate function,
  `attachFinallyV`, to align with the spec.

- Renaming local variables to match variable names in the spec.

- Adding comments that refer to specific spec language.

- Adding parenthetical "OPTIMIZATION:" comments to document
  differences between the implementation and spec that are for the
  purpose of efficiency and don't affect behavior.

- Adding parenthetical "UNSPECIFIED:" comments to documented
  differences between the implementaiton and spec that reflect flow
  analysis features that haven't been documented yet.

- Adding comments that directly reference the `attachFinally` and
  `attachFinallyV` sections of the specification.

- Reordering logic to more closely follow the structure of the
  specification.

This is a pure refactoring and does not change the behavior of flow
analysis.

Change-Id: I6a6a6964bd76b87102facf4251b195e5604c7c32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/445780
Reviewed-by: Erik Ernst <[email protected]>
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441