fix: relax Emitter generic constraint to object for TS strict compat

[xusuxiang8]

Fixes #209

Change the Emitter generic constraint from Record<EventType, unknown> to object so that user-defined Events types without a string index signature satisfy the constraint under TypeScript strict mode.

Example:

// Before: Type 'Events' does not satisfy the constraint 'Record<EventType, unknown>'
interface Events {
  foo: string;
}
const emitter: Emitter<Events> = mitt<Events>();