chore: mark optional peerDependencies as optional

[favna]

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

A while ago a RFC was made for yarn to allow marking peer dependencies as optional and with that silencing the warnings about missing peer dependencies. You can read that RFC here. This was added in yarn in version 1.13.0 (PR and pnpm version 3.2.0-1 (PR). Later it was also added to npm in version 6.11.0 (Changelog)

Since it is now available in all 3 major package managing CLI tools I think this repo should make use of the additional fields in package.json to silence the warnings.

Note: a similar PR has been made to the Commando repository: discordjs/Commando#278

Status

• 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

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.