Allow listeners passed into runtime.onMessage.addListener to return void

[namukang]

Previously, listeners for runtime.onMessage.addListener could return Promise<unknown> | true | void:

webextension-polyfill-ts/out/namespaces/runtime.d.ts

Line 584 in b6e8bd1

) => Promise<unknown> | true | void

Currently, they require a return value of Promise<unknown> | true | undefined:

webextension-polyfill-ts/out/namespaces/runtime.d.ts

Line 583 in 75f8b24

) => Promise<unknown> | true | undefined

Commit where change was made: 85437fc

I believe the correct type should be void rather than undefined so that listeners can return without having to specify undefined.