Make a function acceptable for :skip_code_autolink_to option

[irisTa56]

Currently, the :skip_code_autolink_to option only accepts a list.
This approach becomes impractical when dealing with a large number of terms.

For example, I'm working on a project where some hidden modules are automatically defined, and some documentation tries to link to those modules (to be precise, types defined in those modules).
My problem there is that the number of those terms is too many to list in :skip_code_autolink_to option.

So it is useful if :skip_code_autolink_to option accepts regular expressions or, more flexibly, a function.
I choose the function approach as it aligns with the discussion in #1759 (comment).
(I'm pleased to implement the same feature for :skip_undefined_reference_warnings_on option if this PR is approved.)

As a caveat, because ExDoc.Autolink is internal (@moduledoc false), I just removed code that expects a list for the skip_code_autolink_to field.
Should I keep backward compatibility at ExDoc.Autolink level too?

[josevalim]

Adding similar to skip_undefined_reference_warnings_on will be welcome!

[josevalim]

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