Allow modification of source code from inside of Enso

[JaroslavTulach]

There are two important abilities of language server which require access deep into the engine internals that we'd like to expose to (privileged) Enso programs:

• control execution & deliver visualizations (addressed by Instrumenter to observe behavior of nodes with UUID #7833)
• text editing operations

This issue proposes to create a builtin to expose access to module Source and allow its modifications from inside of Enso (enabled by a CLI option). By having such builtin, the IDE could submit execution requests that modify the source code. Or modify the sourcecode directly from the y-websocket server.

[jdunkerley]

On hold until the yjs server is running withing GraalJS