Add mergewith[!](combine, dicts...)

[tkf]

Quoting README:

• mergewith(combine, dicts...) and mergewith!(combine, dicts...) are like merge(combine, dicts...) and merge!(combine, dicts...) but without the restriction that the argument combine must be a Function ([#34296]). (since Compat 3.9.0).

Ref: JuliaLang/julia#34296

[martinholters]

Code looks fine.

[tkf]

@martinholters Thanks for the review! I updated the patch.

[martinholters]

LGTM now. Will merge soon unless someone objects or beats me to it.