Alacritty: Implement "modifyOtherKeys"

[dnkl]

In short, enabling _modifyOtherKeys_ lets the client recognize e.g. Ctrl-.; i.e. to see modifiers for keys where modifiers are normally ignored.

Note that in XTerm, the user can also enable this by default via XResources.

The enabling/disabling sequence is:

CSI > Pp m
          Set/reset key modifier options, xterm.  Set or reset resource-
          values used by xterm to decide whether to construct escape
          sequences holding information about the modifiers pressed with
          a given key.

          The first parameter Pp identifies the resource to set/reset.
          The second parameter Pv is the value to assign to the
          resource.

          If the second parameter is omitted, the resource is reset to
          its initial value.  Values 3  and 5  are reserved for keypad-
          keys and string-keys.

            Pp = 0  ⇒  modifyKeyboard.
            Pp = 1  ⇒  modifyCursorKeys.
            Pp = 2  ⇒  modifyFunctionKeys.
            Pp = 4  ⇒  modifyOtherKeys.

          If no parameters are given, all resources are reset to their
          initial values.

The values that can be "assigned" for _modiyOtherKeys_ are:

modifyOtherKeys (class ModifyOtherKeys)
               Like modifyCursorKeys, tells xterm to construct an escape
               sequence for ordinary (i.e., "other") keys (such as "2") when
               modified by Shift-, Control-, Alt- or Meta-modifiers.  This
               feature does not apply to special keys, i.e., cursor-, keypad-,
               function- or control-keys which are labeled on your keyboard.
               Those have key symbols which XKB identifies uniquely.

               For example, this feature does not apply to special control-
               keys (e.g., Escape, Tab, Enter, Backspace) Other control keys
               (e.g., Control-I, Control-M, Control-H) may send escape
               sequences when this feature is enabled.

               The default is "0":

               0    disables this feature.

               1    enables this feature for keys except for those with well-
                    known behavior, e.g., Tab, Backarrow and some special
                    control character cases which are built into the X11
                    library, e.g., Control-Space to make a NUL, or Control-3
                    to make an Escape character.

                    Except for those special cases built into the X11 library,
                    the Shift- and Control- modifiers are treated normally.
                    The Alt- and Meta- modifiers do not cause xterm to send
                    escape sequences.  Those modifier keys are interpreted
                    according to other resources, e.g., the metaSendsEscape
                    resource.

               2    enables this feature for keys including the exceptions
                    listed.  Xterm ignores the special cases built into the
                    X11 library.  Any shifted (modified) ordinary key sends an
                    escape sequence.  The Alt- and Meta- modifiers cause xterm
                    to send escape sequences.

               The Xterm FAQ has an extended discussion of this feature, with
               examples:
               https://invisible-island.net/xterm/modified-keys.html

Broken out from https://github.com/jwilm/alacritty/issues/3091#issuecomment-565715265, and https://github.com/jwilm/alacritty/issues/2688#issuecomment-516987534
@AmaiKinono

[chrisduerr]

Thanks for creating a separate issue for easier tracking.

Note that in XTerm, the user can also enable this by default via XResources.

I think we should be good with skipping this. If there's an escape to set/unset it, I feel like we should be good without a dedicated config option.