refactor!: fix several issues with /ws incorporation

[Qjuh]

Please describe the changes this PR makes and why it should be merged:

#10420 introduced some bugs that get fixed with this:

• Client#shard registers listeners on Client#ws so it needs to be initialized after Client#ws
• Client#ping was NaN before the first heartbeat from the gateway was received, now it'll return null instead
• erroneous rebase of refactor!: fully integrate /ws into mainlib #10420 caused deprecated DiscordjsErrorCodes to reappear in src/errors/ErrorCodes.js

Fixes #10555.

BREAKING CHANGE: Client#ping is nullable now

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

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