Have extensionKind: "ui" only run in the ui

[alexdima]

• In September, we made "extensionKind":"ui" run on the workspace extension host if the extension is only installed there.
• In October, we have added support for "extensionKind": ["ui","workspace"] to express that an extension can run on either side, but prefers the ui side.
• In October, we have left "extensionKind":"ui" to be able to execute on the workspace side.

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We should clean this up and enforce that "extensionKind":"ui" only allows an extension to run on the ui.

[alexdima]

fyi @sandy081 @egamma - I have removed that ui extensions are allowed to execute on the remote side.

Still left to do:

1. extensions actions in the viewlet need to also enforce this everywhere and we need to bring back the tests that we removed in September (go back to the August state) @sandy081 .
2. do a pass over extensions in the marketplace or in our product.json which use "extensionKind":"ui" and check, very likely they might need to adopt "extensionKind": ["ui","workspace"] in their package.json or we should do it for them if we run out of time in our product.json.

@sandy081 @egamma LMK if you need any help with point 2, otherwise I consider this done from my side.

[sandy081]

Added code to disable UI extension on Web. Also disallowed installing them.