Port mystmd + myst-theme to bun

[stefanv]

This is a discussion and tracking issue for porting mystmd to bun.

We explored switching over to pnpm as well, which I initially favored because it is managed by an open source community. However, I trialed both (jupyter-book/myst-theme#782 and jupyter-book/myst-theme#781) and found that bun is faster, and also more precise (it doesn't copy npm's package hoisting behavior).

The team, overall, has had good experiences with bun, so after discussions on discourse (and Rowan struggling to get the React 19 port working with npm), we decided to move in that direction.

[choldgraf]

We discussed this a bunch in Discord and based on the conversation there, I'm +1 on doing this!