[FEATURE] generate Signature in component blueprints

[chriskrycho]

Since RFC 0748, Glimmer components support a Signature type which specifies any arguments they accept, any blocks they yield, and the element types to which ...attributes are applied. Incorporate the signature when generating TS blueprints for Glimmer components, including template-only components. Do not generate them for Ember (classic) components, since there are severe limitations in using classic components that way, and since we only support the Octane programming model in our TypeScript support per RFC 0800.

Unblocks #20352 and thus #20177!

[chriskrycho]

Odd, I wonder why that didn't fail locally? I'll dig in and get that sorted.

[chriskrycho]

Fixed this up to do the right thing when in a non-TS context!

For posterity: The problem was that I updated what <%= defaultExport %> compiles to in the component blueprint. And that gets used across both blueprints and blueprints-js, where the latter is the fallback behavior we introduced to allow Ember to ship blueprints in TS while being used with versions of Ember CLI earlier than 4.3.0, where CLI landed support for TS.

Since the path we’re testing in the JS blueprints is “do we fall back correctly when we can’t do a transform and so have a .js file blueprint input?”—and we do! @cafreeman's work there was correct!—it… doesn’t transform anything. 😂 But since the new defaultExport value was something like export default class Foo extends Component<FooSignature> {}, the test correctly complained.

The solution here was to make these blueprints just a bit smarter: use the fact that we know whether we can compile TS out of the blueprints or not (we already had that capability) to emit different things in the cases where we need to—namely, here, by not emitting a signature type.

We could in the future, and very possibly should in the future, emit a JSDoc comment for the Signature in the case where we are emitting JS, which would provide useful norms for the ecosystem and useful tooling feedback as well. (If you're reading this and want to implement that, let us know—it should be straightforward now that these pieces are in place!)