snap: add option `border_overlap` and other improvements (#8289)

* snap: add option `border_overlap` and other improvements

I really liked the way borders used to overlap when snapping and how
only the window's main surface would snap to the monitor, so I would
like to bring that behavior back, but in the form of a config option.

Other improvements include:

- reduced the number of snap functions from 4 down to 2, and only
one ever gets called at any given time.

- border size should not be added to gap size. It seemed like the
right thing to do at the time, but it makes snapping feel way
stronger than it actually should.

- all const variables have been given the all-caps naming convention.

- to avoid excessive casting, border size is declared as a double.

- to avoid excessive x + w, y + h calculations. I'm using a struct
called Range and working only with start and end values until the
very end of the function.

- check for both monitor snapping as well as reserved monitor space
snapping in a relatively efficient way.

* snap: always border-align for corners and reserved monitor space

We probably don't want to treat reserved monitor space as if it were just
a smaller monitor. Instead, it should be treated more like a borderless
window, which means our window's border should never encroach upon it.