Improve single-file generation

[avivkeller]

See comment posted by @aduh95 in nodejs/node#57343 (comment)

We don't really need to completely generate each file every time, so what if we added a --select <glob>, which allows the user to include the entire context (via --input), but only select specific files for actual generation.

Alternatively,

• --only-regenerate-changed, which compares the timestamps of the input to the output
• --context, which explicitly has files to be context, but not generated

[aduh95]

• --only-regenerate-changed, which compares the timestamps of the input to the output

IMO that's not the correct way forward, that's already what make is doing, so it wouldn't be wise to reimplement it. We should either use make upstream – and we "just" have to make sure running several instances in parallel doesn't break anything. An alternative would be to have a --cache out/doccache.json or something like that, which would contain the hash of the input+output so we can skip if either file has changed – but again, IMO that's overengineering, I'd prefer to let make handle it (and I struggle to see the reason why not).