Add getExecOutput function (#814)

* Add getExecOutput function

* Add tests for exec output

* Modify tests to not rely on buffer size, but only test larger output

* Handle split multi-byte characters + PR feedback

* Fix tests

* Lint

* Update how split byte are sent for tests

__tests__/scripts/stdoutoutputlarge.js

@@ -0,0 +1,3 @@
+//Default highWaterMark for readable stream buffers us 64K (2^16)
+//so we go over that to get more than a buffer's worth
+process.stdout.write('a\n'.repeat(2**24));

__tests__/scripts/stdoutputspecial.js

@@ -0,0 +1,5 @@
+//first half of © character
+process.stdout.write(Buffer.from([0xC2]), (err) => {
+    //write in the callback so that the second byte is sent separately
+    process.stdout.write(Buffer.from([0xA9])) //second half of © character
+})