Create custom panes using editor scripts

**Is your feature request related to a problem? Please describe (REQUIRED):**
We want to be able to create custom panes using editor scripts. Example use-cases:
1. Moving panes to extensions, e.g., changes view as a git pane
2. Asset portal
3. Terminal

**Describe the solution you'd like (REQUIRED):**
Editor scripts can define custom panes in the editor

**Describe alternatives you've considered (REQUIRED):**
Haven't considered any

**Additional context (OPTIONAL):**
Most probably blocked by #8044

The issue names no files, tests, or entry points; start by reviewing the dependency on issue #8044 and the editor's existing pane architecture. Done means editor scripts can define custom panes, including the requested extension use cases, with the supported behavior documented or tested.

Written by the indexing model from the issue text.