[VIM] Inconsistent style between Search and IncSearch

[israelst]

Hi @raphamorim, I know that it is possible to style IncSearch and Search differently, but I would expect these two styles to be the same.

Besides, I prefer the one you have chosen for IncSearch; it provides a stronger contrast than just an underline, which is desirable since it is the style for things we are looking for.

So, what do you think about replacing this:

For this:

If you agree, I would be happy to send a PR for that.