Path Editing, Improve Mathblock Output

Follow-on to [#6326](https://github.com/hashgraph/guardian/issues/6326) ("Path Editing, Improve Mathblock Input" — closed, shipped in 3.7.0), which added direct pathname editing for MathBlock **INPUT** variables. That work only covered inputs; this story extends the same path-based editing capability to MathBlock **OUTPUT** variables.

### Description
When working with the mathblock and adding OUTPUT variables, I want to be able to directly edit the pathname, so that they can be quickly updated and I can avoid using the click-through tree navigation / selection tool when I already know the path name or just need to make a small edit.

### Acceptance Criteria
- Output variable path name is editable
- Pressing '.' character brings up autocomplete menu
- Display validation error if pathname does not exist (but allow invalid entries to be saved)

Reference (completed inputs work): https://github.com/hashgraph/guardian/issues/6326