Add "preserveScroll" option to select.vertically

[forivall]

When using ctrl+u/ctrl+d in helix, unlike kakoune, the scroll position is preserved, such that, if your cursor is at the top of your window, after using ctrl+u, your cursor will still be at the top of your viewport after scrolling up. (in kakoune, your cursor will be at the bottom of the viewport) (helix does have the same behaviour as kakoune after using pgup/pgdn)

My current workaround is to use the following bindings:

  { "key": "ctrl+u",   "command": "dance.run", "args": {"commands": [["dance.select.vertically", { "direction":  -1, "by": "halfPage", "shift": "jump" }], ["dance.view.line", { "at": "center" }]]}, "when": "editorTextFocus && dance.mode == 'normal' || editorTextFocus && dance.mode == 'select'" },
  { "key": "ctrl+d",   "command": "dance.run", "args": {"commands": [["dance.select.vertically", { "direction":  1, "by": "halfPage", "shift": "jump" }], ["dance.view.line", { "at": "center" }]]}, "when": "editorTextFocus && dance.mode == 'normal' || editorTextFocus && dance.mode == 'select'" },