Allow to pass `FixestMulti` object to `summarize.summary()`, `summarize.tidy()` and visualisation function

[s3alfisc]

Currently, all summary and vizualisation functions only accept (mixed) lists of objects of type Feols, Feiv and Fepois, but they do not accept objects of type FixestMulti.

One easy way to change this would be to implement a method for FixestMulti that collects all estimated models in a list. When an object of class FixestMulti is then passed to e.g. summarize(), this method could be employed and summarize should work out of the box on the list of e.g. models of type Feols.

I.e. we'd have

FixestMulti.tolist()
returns [Feols instance, Feols instance, etc]