a few miscellaneous fixes and improvements (#1105)

* add binding for more primitive backspace

* support selecting page up and page down

* fix matchbraceleft for braces that start on x=0

* fix multiline copy-paste indenting

let's say you have two lines like

  <space><space>line1
  <space><space>line2

so you start from cursor x=0 and select both lines, then paste.
we don't want any leading whitespace in this case, because the
cursor is already at x=0 and the selection already includes
whitespace.

runtime/help/keybindings.md

@@ -187,6 +187,8 @@ Start
 End
 PageUp
 PageDown
+SelectPageUp
+SelectPageDown
 HalfPageUp
 HalfPageDown
 StartOfLine
@@ -352,6 +354,7 @@ CtrlRightSq
 CtrlCarat
 CtrlUnderscore
 Backspace
+OldBackspace
 Tab
 Esc
 Escape