Add dotnet tools manifest to dependabot (#1030)

Adds `/.config` to the `nuget` ecosystem directories in
`.github/dependabot.yml` so that Dependabot tracks the dotnet local
tools manifest at `.config/dotnet-tools.json` (currently containing
`dotnet-ef` and `trx-to-vsplaylist`).

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: BenjaminMichaelis <22186029+BenjaminMichaelis@users.noreply.github.com>

Author: Copilot

.github/dependabot.yml

@@ -13,6 +13,7 @@ updates:
   - package-ecosystem: "nuget" # See documentation for possible values
     directories:
       - "/"
+      - "/.config"
     registries:
       - nuget-azure-artifacts
     schedule: