When a subfolder uses `.sync-exclude.lst` file the global one is ignored

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## Feature request

**Which Nextcloud Version are you currently using:** 33.0.5 (see administration page)

**Is your feature request related to a problem? Please describe.**
When I create a .sync-exclude.lst in a subfolder - to ignore a credentials file for example the global one is ignored and I have to paste the whole file again (in that subfolder)

**Describe the solution you'd like**
Add an option in the UI (or settings) to join both files (so the global + from the subfolder) and to add ! if you want to override the global one.

**Describe alternatives you've considered**
My alternative is to paste the global file in the directories but this is a lot of work (I have 50+ dirs :<)

**Additional context**
For example, in my global file I have:
```
*venv*
.git
.env
__pycache__
```

and when I create one in subfolder to ignore .credentials file all of my venvs are synced!