This introduces a new type Slot<ContextType, SlotType> that is used
for enum values used to access context slots.
Together with new types for the various custom contexts used in
Torque, this results in fairly type-safe access to context slots,
including the NativeContext's slots.

Drive-by changes:
- Introduce a new header file to specify headers needed for
  generated CSA headers, to reduce the amount of includes specified
  in implementation-visitor.cc
- Port AllocateSyntheticFunctionContext to Torque.

Bug: v8:7793
Change-Id: I509a128916ca408eeeb636a9bcc376b2cc868532
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2335064
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Seth Brenith <seth.brenith@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#69249}