Add a watch mode

[jeffa5]

It would be nice to have the ability to run something like tectonic -X build --watch with maybe -w for short.

I'd expect this to watch all input files (e.g. working out those from \inputs) used in compilation and then rebuild when those change.

Probably fits more with the new cargo-like cli.

[pkgw]

Yes, this is exactly the sort of elaboration that the new Cargo-like CLI is supposed to help enable! I think a tectonic watch would not be at all unreasonable.

[kpym]

I want to advocate for this feature. If you combine it with precompiled header (that is recompiled only if it change) this can make XeLaTeX a WYSIWYG ;)