Add a "watch mode" to compile task

[frangio]

I would love to have a buidler compile --watch command that I can leave running in the background to recompile contracts when they change on disk.

I would use this to run tests in watch mode at the same time. (Side note: I don't use the built in buidler test command.)

[fdietze]

As a workaround, I'm using the tool entr for watching:

find contracts -iname '*.sol' | entr -cnr npx hardhat compile

[valle-xyz]

up