[REQUEST]: Multiline comment keybinding

### Is your feature request related to a problem? Please describe

During **ispc** compiler exploration I've noticed that there is no keybinding to toggle multiline comment/uncomment action. You can comment/uncomment selections using "Ctrl + /" shorcut, but it triggers each line to become a single line comment

Code example that I've been studying (that's the default code example):
```ispc
// Type your code here, or load an example.
varying int square_even(varying int num) {
return (num % 2 == 0)? num * num: num;
}

// Uncomment this if you would like to execute the code
/*
extern "C" uniform int main() {
print("Hello, World!\n");
print("programCount = %\n", programCount);
print("square_even(%) = %\n", programIndex, square_even(programIndex));
return 0;
}
*/
```

### Describe the solution you'd like

Add "Ctrl + Shift + /" keybinding for multiline comment/uncomment action

### Describe alternatives you've considered

I don't see direct alternatives. Maybe someone would suggest

### Additional context

I might try to implement it by myself, but I need a little bit info where to start my research on keybindings in this repo