improved rubbing out commands with modifiers

* This was actually broken if the command is preceded by `@` and `:` characters, which
  are __not__ modifiers.
  E.g. `Q:@I/foo^W` would have rubbed out the `:` register as well.
* Also, since it was all done in teco_state_process_edit_cmd(),
  it would also rub out modifier characters from within string arguments,
  E.g. `@I/::^EQ^W`
* Real commands now have their own ^W rubout implementation, while the generic
  fallback just rubs out until the start state is re-established.
  This fails to rub out modifiers as in `@I/^W`, though.
* Real command characters now use the common TECO_DEFINE_STATE_COMMAND().
* Added test cases for CTRL+W rub out.
  A few control characters are now portably available to tests
  via environment variables `$ESCAPE`, `$RUBOUT` and `$RUBOUT_WORD`.

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