add support for models of type lm, ivreg, fixest and felm passed to rwolf as lists

[s3alfisc]

• [ ] - lm
• [ ] - ivreg
• [ ] - lfe
• [x] - fixest

passed to rwolf as a list()

[s3alfisc]

This should in general be easy as rwolf() accepts inputs of list().

What needs to be added:

• [ ] adjust get_stats_fixest() to work with other input types than fixest
• [ ] check if any other logic in the code depends on objects of type fixest

E.g., currently, rwolf() does not require to specify how to conduct inference - it simply fetches that information from the fixest objects. Hence to support objects of type lm and ivreg, I would have to re-introduce the clustid function argument.