chore(typings): declaring explicit nullable returns (discordjs#3134)

* strict nullable & WebSocketManager private typings

* missing semicolon

* kyra suggestion #1

Co-Authored-By: izexi <43889168+izexi@users.noreply.github.com>

* kyra suggestion #2&3

* space's requested changes

* space's requested change

* strict nullable & WebSocketManager private typings

missing semicolon

kyra suggestion #1

Co-Authored-By: izexi <43889168+izexi@users.noreply.github.com>

kyra suggestion #2&3

space's requested changes

space's requested change

* resolve conflicts

* deflate