Add unit tests for line counter (#1444)

I'm curious if not using `require('os').EOL` for splitting lines in lineCounter is problematic or not. Since I don't have a Windows machine I'm testing this in your CI! :D

test/lineCounter.js

@@ -0,0 +1,18 @@
+const fs = require('fs');
+const assert = require('assert');
+const lineCounter = require('../src/utils/lineCounter');
+
+describe('line counter', () => {
+  it('counts number of lines of a string', () => {
+    const input = ` line 1
+      line 2
+      line 3`;
+
+    assert(lineCounter(input) === 3);
+  });
+
+  it('counts number of lines of a file from disk', () => {
+    const input = fs.readFileSync('./test/lineCounter.js').toString();
+    assert(lineCounter(input) === 19);
+  });
+});