Add ability to specify the app path for react-scripts

[walidghallab]

Is your proposal related to a problem?

I am unable to use react-scripts when the app path is different from where I am running the script.

Describe the solution you'd like

We can add environment variable (e.g. REACT_APP_PATH), where it is used as the app directory when specified and use the current directory otherwise (current behavior).

From the code I can see similar solution was applied for BUILD_PATH.

I can send a PR if this proposal gets approved.

Describe alternatives you've considered

I've considered adding it as a command line, however didn't do it bec:

• Adding env variable is consistent with the current behavior (e.g. BUILD_PATH)

Additional context

Code for the existing BUILD_PATH env: link