handle returned assignments in key events

[commy2]

When merged this pull request will:

• Atm. keybindings that return assignments error:

Test script (15 is tab key):

["Test001", "Action001", "Test Text", {
    systemChat str diag_frameNo;
    0 = 0
}, {}, [15, [false, false, false]], false] call CBA_fnc_addKeybind;

Before: generic error in expression
After: works, assignment treated as "false" (like any other non boolean return value).

[dedmen]

0 would be faster than nil but would ofc define _x, doubt that would break anyones code?

[commy2]

_x previously leaked into the user's code block alongside all the other locals like _params, _code and _keybindParams etc.
One could also use [_x] I suppose to keep the _x the same.
I just don't care about micro optimizations like that. The code is still a fifth of what it used to be before my rewrite and no one ever complained about it.
_x being nil was just the logical thing, as in "this variable I never defined is undefined", from the users perspective.