Support node scripts in BROWSER

Created by: GAumala

Modify OpenBrowser.js to run node scripts specified with the BROWSER environment variable as discussed in #1533 (closed) . If the value of the BROWSER environment variable ends with '.js' a child process is spawned to execute the script with node.js. The command executed in the child process is 'node PATH_SCRIPT'.

Some observations I'd like to discuss:

• Since BROWSER now does more than just opening your browser, maybe we should consider renaming a few things.
• The string argument of the script to run with node is not sanitized in any way. Nothing is checked, the string is passed to the child process as is. I don't know what is the worst thing that could happen. Scripts ending in '.JS' are not executed.
• The openBrowser() function returns a boolean. I decided to stick with this behavior, but I noticed that the result is not used in react-scripts/scripts/start.js.

How to test this:

create any 'script.js' file. You can use this one that writes a file to the current working directory:

const fs = require('fs')

fs.writeFileSync('message.txt', 'script says hello!');

Now in the same directory execute:

BROWSER=script.js npm start

The file should be created in that directory and no browser should be opened.

And of course, the other use cases of BROWSER still work

BROWSER=none npm start
BROWSER=firefox npm start

Any feedback is appreciated.