Use `@tagName` only if `ember-decorators` dependency exists

[Turbo87]

While tagName = '' is only semi-officially supported, @tagName() appears to be the better alternative. Unfortunately it needs an additional dependency though, which some projects might prefer to avoid.

I would propose to check the project dependencies (similar to how we do it for ember-component-css), and only use @tagName when the project already has a dependency on ember-decorators. Additionally we could make it possible to override the detection with a CLI option, if necessary.

@simonihmig thoughts?