bug#47599: 28.0.50; Feature request improve/update isearch

[Ergus]

Hi:

Just to follow this:
https://lists.gnu.org/archive/html/emacs-devel/2021-04/msg00080.html

I open a feature request to suggest some updates in the isearch code and
the addition of some simple functionalities like:

1) Option or command to automatically go to the other end on exit.

2) Convert isearch-wrap-function, isearch-push-state-function,
isearch-filter-predicate into customs instead of use defvar, improve a
bit their documentation and provide some handy options.

3) Make some general refactor of isearch code to simplify and remove
some redundant or useless checks, vars and code. Update the code to use
some useful modern api like define-minor-mode or easy-mmode-defmap. 

There are some external packages that reimplement the isearch
functionalities or hack it to produce some of these
functionalities/behavior and maybe (as an extra) we could consider add
some customs to make isearch behave like a bit like that; because most
of them are not very different to what isearch already does now.

Example: https://github.com/raxod502/ctrlf 

Some of the differences where mentioned already here:

https://lists.gnu.org/archive/html/emacs-devel/2021-04/msg00108.html