navigation
(ruby/reline#677)

Fixes ruby/reline#675

This commit extracts the upward navigation condition in `LineEditor#input_key` to a new private method, and adds a new alias. This change allows Reline to support upward navigation in when a user has configured `inputrc` to map Shift-Tab to `menu-complete-backward`, a common setting in Bash (>= 4.x).

Instead of special-casing upward navigation in `LineEditor#input_key`, we now allow it to be processed by the branch that calls `process_key`. The extracted method no longer includes the editing mode check since this check is already made by `#wrap_method_call` by the time `#completion_journey_up` (or `#menu_complete_backward`) is called. Since upward navigation is happening in a method other than `#input_key` now, the `completion_occurs` variable that used to be local to `#input_key` is changed to an instance variable so that the new method can change its value. (I see many examples of mutating such instance variables in `LineEditor`, so I assumed this would be an uncontroversial change consistent with the coding practices already in place.)

Test coverage of this change has been added to the emacs and vi `KeyActor` tests.

Many thanks to @ima1zumi for their very helpful comments on #675 which encouraged me to contribute this work!

ruby/reline@2ccdb374a4

* master: (29 commits)
  Not all `nm`s support the `--help` option
  Emit a performance warning when redefining specially optimized methods
  YJIT: A64: Avoid intermediate register in `opt_and` and friends (ruby#10509)
  [ruby/reline] Remove not implemented method (ruby/reline#680)
  [ruby/reline] Fix vi_to_column which was broken (ruby/reline#679)
  Include more debug information in test_uplus_minus
  [Universal parser] DeVALUE of p->debug_lines and ast->body.script_lines
  Add more assertions in `test_uplus_minus`
  `super{}` doesn't use block
  fix incorrect warning.
  Update default gems list at fc36394 [ci skip]
  [ruby/optparse] bump up to 0.5.0
  show warning for unused block
  Emit `warn` event for duplicated hash keys on ripper
  [ruby/reline] Refactored Default Key Bindings (ruby/reline#678)
  [ruby/reline] Refactor waiting_proc and waiting_operator_proc (ruby/reline#649)
  [pty] Fix missing `or`
  [pty] Fix `ptsname_r` fallback
  [ruby/irb] Allow defining custom commands in IRB (ruby/irb#886)
  [ruby/reline] Support `menu-complete-backward` command for upward navigation (ruby/reline#677)
  ...