Add automatic area labeling

**Is your feature request related to a problem? Please describe.**
After #7686 adds `area-*` labels, maintainers will still need to select an area for each new issue.

**Describe the solution you'd like**
Use [`dotnet/issue-labeler`](https://github.com/dotnet/issue-labeler), which is already used by `dotnet/runtime`, to add one `area-*` label to new issues. Add `needs-area-label` when the model cannot select an area, and keep existing area labels unchanged. Train and review the model before automatic labeling is enabled.

**Describe alternatives you've considered**
Use an LLM-based labeler if the existing model does not give sufficiently accurate results.

**Additional context**
This work depends on #7686.

Start by reviewing issue #7686 and the dotnet/issue-labeler setup already used by dotnet/runtime. Determine how to train and review the model, configure automatic labeling for new issues, preserve existing area labels, and add needs-area-label when no area is selected. Done means the model has been reviewed and the automation is ready to enable.

Written by the indexing model from the issue text.