Specifying wheelchair_boarding for a platform which isn't accessible from outside, but accessible for interchange purpose

### Describe the problem

The current specifying on the `wheelchair boarding` says that:

> For child stops:
> 0 or empty - Stop will inherit its wheelchair_boarding behavior from the parent station, if specified in the parent.
> 1 - There exists some accessible path from outside the station to the specific stop/platform.
> 2 - There exists no accessible path from outside the station to the specific stop/platform.

Unfortunately it isn't possible to specify that it is possible for wheelchairs to change trains even if it is not possible for wheelchairs to get out of the station.

I would need a way to specify the wheelchair accessibility islands within the station.

### Use cases

I want my journey planner to tell wheelchairs to change trains between the platforms of the same direction at Stockwell or Finchley Road.

### Proposed solution

Add a field `wheelchair_accessible` in `pathways.txt` with the same enum values of `wheelchair_boarding` in `stops.txt` and `wheelchair_accessible` in `trips.txt`.

|`wheelchair_accessible`|meaning|
|---|---|
|`0` (default)|No information is known if this pathway is wheelchair accessible|
|1|This pathway is wheelchair accessible|
|2|This pathway is not wheelchair accessible |

If there exists a sequence of pathways where `wheelchair_accessible` all have a value of `1` between a pair of platforms, wheelchair interchange between these platforms is possible, even if the platforms have a `wheelchair_boarding` value of `2`.

### Additional information

_No response_