many keyboards have right & left keys for ctrl/alt/meta(e.g. windows/command).  the only support we have in hterm today is via alt-gr-mode which turns the right alt key into a compose key.  this could be more useful to users when the OS/browser provides shortcuts using ctrl/alt/meta that Secure Shell would otherwise be snarfing -- macOS examples are legion (unicode input, window managers, etc...).

we should however expose this at the keybindings level, and by default make it so the right side keys are ignored by hterm.  this way people can set up keybindings like:
  Alt-M: do something when left or right Alt-M is pressed
  LeftAlt-M: only do something when the left Alt-M is pressed
  RightAlt-M: only do something when the right Alt-M is pressed

although this syntax is a little limited as there are more possible location values:
  https://developer.mozilla.org/en-US/docs/Web/API/KeyboardEvent/location

notably, numpad, and maybe in the future more ?  perhaps going with the existing syntax:
  [Location-][Modifier-]<key>
  Left-Alt-M

as long as the set of values in Location and Modifier are kept distinct, our grammar should be unambiguous -- we don't allow any <key> to be a Modifier in the current design.