[General] Add configurable env vars globally and per repo.

**Mood:** 😊
**Category:** General

Add configurable env vars globally and per repo.

To workaround #341 I'd like to set `MSBUILDTERMINALLOGGER=off`, but there's no place to configure this within the app.

It would probably be useful for many use cases to be able to configure env vars both globally and per repository and have those apply to terminals and Run commands within sessions.

---
| Field | Value |
| --- | --- |
| App version | 0.2.7 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 2 |

Start by tracing how the app creates terminals and runs commands within sessions, then identify where environment settings could be loaded globally and per repository. Define how both scopes apply to terminals and Run commands, and verify the configured MSBUILDTERMINALLOGGER value is available in each case.

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