Adding (configurable) Line Numbers to the Editor - every 10 lines

[MichaelBaiJie]

• 1. I documented all behaviour within the code as far as I could.
• 2. This PR targets the develop branch and not the master branch.
• 3. I have tested my changes extensively and paid extra attention to potential
  cross-platform issues (e./g. Cmd/Ctrl/Super key bindings)
• 4. I ran the yarn lint and yarn test commands successfully
• 5. I have added an entry to the CHANGELOG.md
• [?] 6. I matched my code-style to the repository as far as possible
• 7. I agree that my code will be published under the GNUP GPL v3 license
• 8. In case I created new files, I added a header to them (see the existing
  files for examples)
• 9. Before opening this PR, I ran git pull a last time to prevent any merge
  issues

Description

I added line numbers to the editor, they appear every 10 lines.

Changes

I added CodeMirror.lineNumbers to the cm options file, added some configurations to the gutter settings in Editor.less, and added a checkbox that toggles the numbers on/off

Additional information

One of the things I couldn't make nicer was the size of the gutter... so the lineNumbers gutter is 7em wide, to account for larger files. The line numbers also match the size of the line it is on, so header line numbers are larger. This is my first pull request, I'd love more guidance on how to improve the changes!

Tested on: <!-- Mac OS ->

[boring-cyborg]

Thank you for opening your first PR! 🎉 We are very happy and would like to thank you very much for your contribution. If everything checks out, we'll make sure to review the PR as soon as possible and give feedback. In the meantime, to make the reviewing process as fast as possible, you can help us by checking the following things:

• Did you follow the JSStandard coding style? - Did you comment everywhere where the necessity of a piece of code or the
  way it was implemented is not immediately obvious?
• Did you attempt to stick as much to current coding habits as possible?
  (Note that this does not apply to pieces of code where we ourselves
  obviously violated good coding practices, which unfortunately happens
  sometimes. But please indicate this in your PR so that we know what you
  rectified!)

Furthermore, make sure that the linter does not complain, which will check your code on every new commit. If the linter task fails, make sure to run yarn lint locally and check the file eslint_report.htm which will tell you precisely what went wrong.
Stay sharp, and thanks again!

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.