Handle forward-delete key in emacs and vi editors

On every modern system, the forward-delete key on PC/Mac keyboards
generates the VT220 sequence ESC [ 3 ~. Every other shell with an
editor handles this now, ksh93 seems to be the last not to.

src/cmd/ksh93/edit/emacs.c: escape():
- Handle the ^[[3 as part of normal escape processing, then read an
  extra character to check for the final '~'. If detected, insert
  an ERASECHAR key event.

src/cmd/ksh93/edit/vi.c: mvcursor():
- Replace the ^[[3~ sequence by an 'x' command. We have to
  special-case its processing, because vi mode parses numbers as
  repetition operators. The escape sequence contains a number,
  making it incompatible with normal command handling. This means
  number repetitions don't work with the forward-delete key. If
  that annoys anyone enough to fix it, a patch would be welcome.
  For now, it will do to make the forward-delete key stop
  exhibiting bizarre behaviour (beep + change case + move forward).

src/cmd/ksh93/sh.1
- Copy-edit emacs documentation for VT220-style sequences; map them
  to their actual key, otherwise it's meaningless to the reader.
- Document the new forward-delete key behaviour for emacs mode.
- Leave the forward-delete key for vi mode undocumented for now, as
  repetitions don't work, so it doesn't really match the vi canon.
  (OTOH, it doesn't work in vim, either...)

NEWS

@@ -8,6 +8,8 @@ Any uppercase BUG_* names are modernish shell bug IDs.
 - Corrected rounding of floating point values by ksh's printf %f formatting
   operator. Fix contributed by @hyenias.
 
+- The forward-delete key now works as expected in emacs and vi editing modes.
+
 2020-09-11:
 
 - The 'command' regular builtin utility (which runs a simple command, removing