Python: Use IPython-like input prompts e.g In [1]

[seeM]

We currently use input prompts like ordinary Python REPLs e.g. >>>.

IPython instead uses input prompts like In [1]. We might want to follow that convention for a few reasons:

1. Some magic commands refer to cell numbers, which are currently hard to identify in Positron (via @isabelizimm)
2. Stack traces may refer to cell numbers e.g. Cell In[1], line 1 (via @isabelizimm)
3. We get to be more consistent with IPython

This is more of a nice-to-have so probably not very high priority for now.

IPython also labels outputs like Out [1] but there's far less value in us following that if we already label inputs.

[ntluong95]

That makes sense as I was confused between Python REPLs and IPython

[ntluong95]

Also see this thread #3802 for more inspiration for the console