changed arg type of EventmitHandler

[ncukondo]

It allows variable args with type.

For example

const event1 = eventmit();
// Register handler
event1.on(() => {
    console.log(1, "value");
});
// Invoke handler
event1.emit();

const event2 = eventmit<{ key: string }>();
// Register handler
event2.on((value) => {
    console.log(1, value);
});
event2.on((value) => {
    console.log(2, value);
});
// Invoke handler
event2.emit({
    key: "value"
});
// Unregister handler
event.offAll();

const event3 = eventmit<[type:string,message:string]>();
// Register handler
event3.on((type,message) => {
    console.log(1, `eventType:${type} message:${message}`);
});
// Invoke handler
event3.emit("customEvent1","some message");

[azu]

Thanks for PR.
I understand your needs.
📝 I found the same issue on mitt developit/mitt#115

However, I want to keep API simple.
The generics keyword is a tuple of types, but EventmitHandler and emit receive arguments as rest elements.
I think that It is hard to make this API instinctive.

// tuple/array types
// tuple is best type because tuple label is useful
const event3 = eventmit<[type:string,message:string]>();
// vs. ...rest element
// tuple is implicitly expanded as rest parameter 
event3.on((type,message) => {
    console.log(1, `eventType:${type} message:${message}`);
});
event3.emit("customEvent1", "some message");

I have not the strong motivation for this. I close this PR.
Thanks.