fix(sdk): explicit type for project template

[sean-perkins]

Current Behavior

When working with the public API methods, such as openProject, it is unclear what the available options for the templates are. When executing these functions, Stackblitz will throw an error on the site about an invalid template, but does not inform developers about available template values.

New Behavior

This PR makes the template value of the Project interface have strict types of the available options. This allows developers to know the available template values when working with the public API.

Other Information

It appears the API documentation: https://developer.stackblitz.com/docs/platform/javascript-sdk/#sdkopenprojectproject-opts could also be updated to reference the current latest list.

[fvsch]

Thanks a bunch for the PR @sean-perkins.

We considered doing this before but opted not too at the time, because we don't want to block users of old versions of the SDK from using a newly supported template type.

That being said,

1. It's a nice DX improvement for user of TypeScript or of IDEs that support TS types in JS (e.g. VS Code).
2. The amount of "blocking" this will do is limited, since:
   • We do have console warnings already for project types that the SDK doesn't know about.
   • JavaScript users won't be affected (no compilations errors blocking a build).
   • TypeScript users can always escape it with template: 'new-template-name' as any.

IMO that's good enough to make the jump and use a more precise type, as per this PR. What do you think @EricSimons?

[fvsch]

Let's get this in, as it'll be released in the 1.7.0-alpha.* channel for now, and we can always go back to a more permissive type if need be before 1.7.0.