Feature request: Need to define key combinations for `trigger`

**Is your feature request related to a problem? Please describe.**

I want to show the layout when I press AltGr + Shift, so I'll need to define two layouts with `"trigger": ["RAlt", "LShift"]` and `"trigger": ["RAlt", "RShift"]` (since "Shift" by itself does not work for some reason here).

`customShiftMappings` does not solve the problem, because it only works when Shift is pressed. I want the combinations of pressing Shift, Ctrl+Shift and AltGr+Shift (which is either Ctrl+Alt+Shift or AltGr+Shift, because AltGr is understood as Ctrl+Alt in most layouts).

For example, pressing Ctrl+Shift+4 shows me the ₹ symbol in English (India) and Malayalam layouts (Also Ctrl+Shift+1 is a Zero-Width Joiner (ZWJ) and Ctrl+Shift+2 is a Zero-Width Non-Joiner (ZWNJ)). Likewise, AltGr and AltGr+Shift show a different set of keys, which — for example, you can see here: [US International (KBDUSX.DLL) | Keyboard Layout Info](https://kbdlayout.info/KBDUSX/)

I don't exactly understand the difference between using `keyCodeMap` and setting a new layout, but I don't think it will help with the goal either.

**Describe the solution you'd like**

I want key combinations to be supported by `trigger`.