Add `this: void` to all methods

[FloEdelmann]

What is the purpose of this pull request? (put an "X" next to an item)

• [ ] Documentation update
• [ ] Bug fix
• [ ] Feature
• [ ] Code style update (formatting)
• [x] Refactoring (no functional changes)
• [ ] CI related changes
• [ ] Other, please explain:

What changes did you make? (Give an overview)

When destructuring an instantiated emitter, the typescript-eslint rule unbound-method reports these cases:

const { emit, on, off } = mitt()
//      ^^^^  ^^  ^^^
// Avoid referencing unbound methods which may cause unintentional scoping of `this`.
// If your function does not access `this`, you can annotate it with `this: void`, or consider using an arrow function instead.

Since this is not accessed inside mitt, this PR adds the this: void annotation to the emit, on and off methods.

Is there anything you'd like reviewers to focus on?

No.

Does this PR introduce a breaking change? (What changes might other developers need to make in their application due to this PR?)

No.