fix: Improve windows support (scripts)

[GordonSmith]

Very opinionated I am afraid - just using what I am used to...

[Fil]

Is this still useful after #944?

[mbostock]

We adopted cross-env for mocha at least.

[GordonSmith]

@Fil - Has #944 been tested in both a git bash shell (on windows) and a cmd prompt? Am happy to close, was just sharing a quick fix I made to see it work OMM.

[Fil]

@GordonSmith if you can test on your side please report any findings—it will be very useful.

[GordonSmith]

@Fil tested with cmd + PowerShell:

1. npm run build

load docs/quakes.json.sh → [missing] 'sh' is not recognized as an internal or external command,
operable program or batch file.
error in 31ms: spawn sh ENOENT

Which would be expected...

2. npm run dev:
   Hangs after tsx watch docs/theme/generate-themes.ts as & does not work as expected.

3. npm run test:
   Fails on build:mocha due to cp not being recognised as a command.

[mbostock]

If cp doesn’t exist then how are the tests passing on Windows? 🤔

https://github.com/observablehq/framework/actions/runs/8141979376/job/22250489360

[GordonSmith]

cp exists in git bash (on windows) and PowerShell, but not the regular command shell...

[GordonSmith]

call	git bash	command prompt	PowerShell
npm run build	pass	fail (expected)	fail (expected)
npm run dev	pass	fail (due to &)	fail (due to &)
npm run test	pass	fail (due to cp)	pass