Zig watch similar to cargo watch

[andrewrk]

Discussion continues at

Initial implementation of `zig build --watch` just landed in master branch

For those adventurous Zig users out there living on master branch, and who happen to use Linux, give the new --watch flag a try, and see if you like it. Windows and macOS support are close, just needs some OS-specific glue code to make it work. I’m fairly confident the abstractions that I made will hold up nicely when it comes to those file system APIs since it’s based on watching directories only. Contributions welcome! The way I see it, the end goal here is having incremental compilation +…