estimate_fun takes list of formulas

[MauKappel]

Let the estimate_fun take a list of formulas instead of a function. Draft close to the code from add_difference and pvalue_fun.

If the draft is okay I can do the TODO:

• Change documentation
• Add to tbl_uvregression
• Similar changes for pvalue_fun

#2058

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[ddsjoberg]

I am going to close this PR for now, and we can re-open if we opt for the change. Thanks