Core Tools should block settings value with invalid name in `local.settings.json`

As the following screenshot, Azure Functions can accept only letters, numbers (0-9), periods ("."), and underscores ("_") as appsetting name.

But core tools can accept other characters from those, like half-width space.

It's better to match both of them.

Start by locating the Core Tools entry point that reads app settings from local.settings.json and check where setting names are validated. Confirm the current acceptance of a half-width space, then make completion mean that names accept only letters, digits, periods, and underscores.

Written by the indexing model from the issue text.