Improve "Remove extra paragraph spacing" feature

[Palloxin]

repost of #1164 due to conflicts (related code moved to another file) Currently, it just removes all <br>. With this change you could: -remove excessive <br> -remove some ~~zero-width spaces and~~   (I found a novel that put them in a p to simulate an extra empty paragraph). -remove a br after the end of a p. It's just extra space. -remove a br present right before the end of a p.

EDIT: the feature code has regex chaining now.

[nyagami]

<br> <p class="haha"> failed this test

[Palloxin]

Double br exception:

[Palloxin]

I think it's ready now EDIT: now it really is.