docs: improve command line environment variable tips

[dy0gu]

What does this PR do?

• Documentation

Improves the existing docs section for setting variables through the command line.

The method currently described in the docs is the following:

$ FOO=helloworld bun run dev

The problem is this syntax is only available on POSIX-compliant shells, which mostly means MacOS and Linux. I assumed that bit was a leftover from when Bun did not support Windows and had yet to be updated.

I've recently seen a significant number of Windows users being confused as to why following the docs on this was not working, so I decided to clarify where that syntax is available.

In addition to that I added a cross-platform alternative for setting variables like previously expected, although I'm open to feedback if you think it's not clean enough to be in the official docs. It makes use of a somewhat chained and lengthy bun exec syntax.

I believe the presence of the examples added by this PR are important as I found myself and other peers regularly needing specific package.json commands to set the NODE_ENV variable, for example.

[dy0gu]

I update the mention of bun exec on Windows to bun shell instead, for clarity.

I also shortened the longer bit of text, I think all the needed info is still nicely there.

Side note, it's nice to finally give back to Bun, the experience has been great and the built-in environment utilities are probably one of the biggest reasons for having migrated personally. Also, thanks for your review @RiskyMH, let me know if any more changes are needed!

[RiskyMH]

nice glad to hear!

to make it more clear i put it in a dropdown which i believe will catch your eye better also.. let me know if you preferred it the other way though.

normal	expanded

[dy0gu]

Looks good to me 👍