Only show project templates by default with "dotnet new --list"

[bekir-ozturk]

Extracting from work item: dotnet/sdk#10587 (comment)

@davidfowl wrote on Feb 2, 2020 (irrelevant sections are removed):

A few observations:

• It's not clear which templates are item vs project templates.

Some suggestions:

• Show project templates by default and have an option to show all templates (or to filter by a specific template type)

[baronfel]

This is made slightly more complex because 'global' items like editorconfig, gitignore, etc. are all currently item templates. Once constraints are in, we will start doing this by default, because items will begin to declare their requirements in terms of constraints, and items that don't satisfy their constraints simply won't be listed.

[vlada-shubina]

Closing. The templates shown in the list can be restricted by constraints instead. Also comment above makes it clear that showing only project templates is not an option.