runtime(netrw): honor `g:netrw_alt{o,v}` for `:{S,H,V}explore`

runtime(netrw): honor `g:netrw_alt{o,v}` for `:{S,H,V}explore`

Make `:Sexplore` / `:Hexplore` / `:Vexplore` commands honor the user
`&split{right,below}` settings (or netrw-specific `g:netrw_alt{o,v}`)
instead of hardcoding a split direction. Similarly, update banged
variants of the two latter commands to follow the inverted preference.

closes: #15417

Signed-off-by: Ivan Shapovalov <intelfx@intelfx.name>
Signed-off-by: Christian Brabandt <cb@256bit.org>