feat: .NET appsettings files in repos

appsettings.json and appsettings.<Environment>.json tracked by git are
listed under Repos, keys named as .NET names them (ConnectionStrings:Default).

- The secret patterns pick the keys, the rest hidden, with
  dotnet.secretPatterns for .NET files' own.
- An App Service Key Vault reference, @Microsoft.KeyVault(…), is marked
  as a reference; a BOM at the start is fine.
- A .NET icon, themeable with dotnetColor.

docs/config.md

@@ -76,6 +76,7 @@ theme:
   repoColor: ["#c099ff"]
   springColor: ["#c3e88d"]               # Spring Boot config files
   composeColor: ["#82aaff"]              # Docker Compose files
+  dotnetColor: ["#c099ff"]               # .NET appsettings files
 ```
 
 A role lazysecrets doesn't know, a typo or one from a newer version, is