Implement multiverse stepwise estimation syntax `mvsw`

[leostimpfle]

pyfixest supports the stepwise and cumulative stepwise multiple estimation syntax (see here). fixest additionally supports a so-called multiverse stepwise multiple estimation syntax mvsw which is currently not implemented in pyfixest.

From the fixest documentation:

mvsw: it stands for multiverse stepwise. It will add, in a stepwise fashion, all possible combinations of the variables in its arguments. For example mvsw(x1, x2, x3) is equivalent to sw0(x1, x2, x3, x1 + x2, x1 + x3, x2 + x3, x1 + x2 + x3). The number of models to estimate grows at a factorial rate: so be very cautious!

[s3alfisc]

Yes, would be cool to have!