Resolve "Calculate diff of the Web IDE file changes"

What does this MR do?

This MR introduces some modules that will be used to generate a diff object for Web IDE file mirroring.

How can I test this?

Apply this patch 20190425_register_mirror_plugin.patch to:

• console.log the diff object when CMD+s is hit while the editor is focused
• console.log the diff object when editor loses focus

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes #10232 (closed)