Prototype visual editor for FLINT

There are a couple of problems with the current flinteditor from a user perspective:
1. It's way too intimidating for the average lawyer
2. Large law texts are hard to browse and check in the sources-array
3. It's relatively hard to explore and see the structure of the full model

The idea is that this can be fixed by creating a visual editor.

[MPS](https://www.jetbrains.com/mps/) is a prime candidate to build such a solution on.

Start by reviewing the current flinteditor and how it exposes sources-array content and the full model structure. Evaluate the proposed JetBrains MPS approach against the three user-facing problems, then define a concrete prototype scope and observable browsing and exploration outcomes before implementation.

Written by the indexing model from the issue text.