Introduce escape/unescape and possibly base64 encode/decode features

While working with JSON, it is often copied from logs or any kind of outputs, and also inserted into configurations or variables.
Because of that, it would be very helpful to have at least escape/unescape features integrated into JSON Editor Online instead of using 3rd party software.
It would be also beneficial to introduce simple base64 encode/decode features as JSON often represents binary payload in all kinds of communications.

I propose simple combo box with several items (operations) that would take currently entered text, perform selected operation and copy result to clipboard:
- escape
- unescape
- encode (base64)
- decode (base64)

No files or tests are named. Start by locating the JSON Editor Online text area, operation controls, and clipboard handling, then review how editor actions are tested. Done means a combo box can apply escape, unescape, base64 encode, and base64 decode to the current text and copy the result to the clipboard.

Written by the indexing model from the issue text.