feat: add bun instructions

[colinhacks]

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

• Adds instructions for bun package manager

Some additional info:

• The guide builds successfully with npm run build locally.
• Discord.js is fully supported in Bun: https://bun.sh/blog/bun-v0.6.7
• I've put in a sister PR that adds bun to the create-discord-bot CLI: https://github.com/discordjs/discord.js/pull/9798

[Jiralite]

Discord.js is fully supported in Bun: bun.sh/blog/bun-v0.6.7

We have a page on how to get started with sharding, which will apparently not work: https://github.com/oven-sh/bun/issues/4159#issuecomment-1678647702.

I personally would also wait for https://bun.sh/1.0 too. Not sure what others think.

[colinhacks]

Oops yeah hadn't seen that.

Definitely reasonable to wait for 1.0. It may be worth noting that all the changes in the PR pertain to usage of bun as a package manager, not a runtime. We're seeing a lot of Node.js users use bun exclusively as a package manager/script runner.

[aefox]

bun 1.0 is out now 👍 https://bun.sh/blog/bun-v1.0