Run prettier on reference files as they are generated

[huonw]

This does a bunch of leg work to have the reference files be reformatted (with prettier) as they are written. This makes the generation notably slower (slightly less than 1s -> slightly less than 4s on my machine), but saves from having to remember to run the formatting separately... which takes just as long as the extra time anyway.

This requires a few steps, broken across commits in a useful way (with some not-strictly-necessary changes), because prettier only has an async API. This seems to require running as an .mjs ("module") rather than .js to be able to use await at the top level.

After getting to using async + prettier, this then makes a few changes that this allows:

• a neater way to write the JSON files
• writing all files concurrently with Promise.all (writing the targets and subsystems concurrently takes the time from ~4.4s -> ~3.8s, doing the others concurrently makes no difference, but makes the code look uniform).

See also: #50 (comment)

[thejcannon]

That's a work of art :chef's kiss: