document a few more keyboard primitives

Dependencies

• [2] 7IDHIAYI rename modifier_down to key_down
• [*] N2NUGNN4 include a brief reference enabling many useful apps

Change contents

• edit in reference.md at line 254
  [4.9581]

  [2.18]
  + * `App.is_cursor_movement(key)` -- return `true` if `key` is a cursor movement

  + key (arrow keys, page-up/down, home/end)

  +

  + * `App.cmd_down()`, `App.ctrl_down`, `App.alt_down()`, `App.shift_down()` --

  + predicates for different modifier keys.

  +

  + * `App.any_modifier_down()` -- returns `true` if any of the modifier keys is

  + currently pressed.

  +