Create Bun.gitignore for use in bun based projects.

[glyn-marsh]

The text in this file was found in .gitignore after running

$ bun init

on bun v1.1.20. The original file claims it was based off https://raw.githubusercontent.com/github/gitignore/main/Node.gitignore, but this was removed from the commit because this is that repository.

Reasons for making this change:

I am a user of Bun (and I love it) and want people to know of bun and it to gain a presence. I hope that creating a .gitignore file here will do that. This can also be a learning material for people new to the runtime.

If this is a new template:

• Link to application or project’s homepage: https://bun.sh/