Validating precedence in template groups

[vlada-shubina]

When working with template groups, one of the most common mistakes is same or missing precedence.

Consider the checks for the template group / when installing the template package to check if the template with same precedence is already installed inside same group.

However valid scenarios are:

• the templates may have same precedence in case language is different
• the templates may have same precedence in case type is different

[vlada-shubina]

This is a nice fit as validation check in scope of https://github.com/dotnet/templating/issues/2623 for authoring tools we may consider an extended check to check known templates in the template group for precedence clashes (for example using search cache for that, or other location for 1st party templates).