Migrate old-styled ITEM templates

[donJoseLuis]

Background

Enable migration of item templates used in Visual Studio based on old template engine and to template.json using constraints. Other item templates already based on template.json (as MAUI and test templates) should be enhanced to use constraints.

Justification

• Customer impact - better feature set for authoring comparing to old template engine
• Engineering impact - no need to support item templates built on 2 different template engines

Work list

• [ ] MAUI templates
• [ ] dotnet test templates
• [ ] old style item templates

[JanKrivanek]

Prerequisity: https://github.com/dotnet/sdk/issues/28915

[vlada-shubina]

This is not yet done. After implementing support of evaluating context for item templates in https://github.com/dotnet/templating/issues/4285, the next prerequisite is to align with VS team for broader support of item templates, including 3rd party templates. VS needs more metadata then dotnet new here, including hierarchy paths in Add new item dialog and default extension. This may be potentially implemented in ide.host.json.

Then we can start to advise teams to do the migration.