Functions emulator should only ignore what is in `functions.ignore`

[samtstern]

[REQUIRED] Environment info

firebase-tools: all

Platform: all

[REQUIRED] Test case

Right now we have the following file watcher in the Functions emulator: https://github.com/firebase/firebase-tools/blob/bd6a23be0ed9a7b06aa178aa66c94e6cb6ce8045/src/emulator/functionsEmulator.ts#L238

It ignores certain files based on best practices, however in firebase.json the developer can customize functions.ignore, we should match that exactly with our file watcher.

The main use case for this is when developers want to use local node modules, we are excluding them by matching all node_modules

[REQUIRED] Steps to reproduce

N/A

[REQUIRED] Expected behavior

See above

[REQUIRED] Actual behavior

See above