Detect when two app settings are entered with the same name

**Is your feature request related to a problem? Please describe.**
We got a strange error when deploying a bicep template for a function app. Something to the effect of:
"InternalServerError. Could not index into a null array."

After a lot of time spent troubleshooting we finally figured out that the problem was that we had two key value pairs with the same key being set in our app settings.

**Describe the solution you'd like**
It'd be a nice productivity feature if Bicep's intellisense would detect when key value pairs for Microsoft.Web/sites -> siteConfig.appSettings objects have duplicate keys.

Start by tracing Bicep IntelliSense for Microsoft.Web/sites siteConfig.appSettings objects. Determine how duplicate key-value pairs are represented and where diagnostics for them belong. Done means duplicate app-setting keys are detected and reported before deployment, with the relevant behavior covered by tests.

Written by the indexing model from the issue text.