Add a way to read all enabled extensions across all extension hosts

[alexdima]

vscode.extensions.all or vscode.extensions.getExtension will only return extensions from the current extension host. The root cause for this is that the return type IExtension<T> exposes the .exports property, which represents the extension's exported API. Since extensions can export synchronous APIs, we can't automatically proxy it to make it available to other extension hosts.

Also, making vscode.extensions.getExtension return an extension running on a different extension host, has the potential to cause breakage in existing extensions.

We could add a new function to be able to get to extensions that are running on other extension hosts.

This came up before e.g. in #82633

cc Johannes Rieken (@jrieken)

[aeschli]

The main motivation for me is to get access to the manifests of all extensions across extension hosts to find contributions such as jsonValidation.

The same issue came up with the new anycode contributions and the htmlLanguage contribution were exploring in #146731.

[jrieken]

• The simple API change is another overload of the vscode.extensions.getExtension-function, like

export function getExtension(extensionId: string, includeAllExtensionHost: boolean): Extension<unknown> | undefined;

• Also add another property for the Extension-type that allows to tell "same-ext-host" extensions apart from "foreign-ext-host" extension. Maybe a boolean like Extension.isDifferentThanThisExtensionHost or some kind of extension host identifier which you can compare with something like vscode.env.thisExtensionHostIdentifier
• Last, a good way to iterate over really all extension descriptions. Something like vscode.extensions.allAll to complement the all property. Or we review existing usages to decide for the breaking semantic change...