feat: Use declare class where possible

[ExE-Boss]

This changes emit for interfaces:

interface Foo : Bar {
	constructor();
};
Foo includes Baz;

From:

declare interface Foo extends Bar, Baz {}
declare var Foo: {
	readonly prototype: Foo;
	new(): Foo;
}

To:

declare interface Foo extends Baz {};
declare class Foo extends Bar {
	constructor();
]

Which has the added benefit of supporting declaration merging for static methods and properties.

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It also prevents static properties from breaking // $ExpectType on constructors, since it just becomes // $ExpecType typeof Foo.

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This shouldn’t be a breaking change as existing consumers will keep working just fine, this just opens up new ways to extends these definitions.

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• Fixes Emit classes instead of vars #222

[saschanaz]

This seems only for manual JSON data, in that case I think those should be fixed instead.

[sandersn]

This is a possibly gigantic breaking change. I created a PR on Typescript, microsoft/TypeScript#39044, so we can run DT and other tests on it.

[sandersn]

We discussed this in the design meeting and decided not to take this change.

• The DOM types are not really classes.
• Static-side merging still only works with namespaces, which is not discoverable (and fewer people over time know how to use namespaces at all).
• So far, for cases where we want to enable static-side merging, we manually introduce an interface for the static side. We would take a PR that generates these interfaces for all class-like types.

[saschanaz]

Added a comment: microsoft/TypeScript#39054 (comment)