Support extensionKind to be an array

[alexdima]

Some extensions can execute only on the ui side, some can execute only on the workspace side, while some can execute in both places and might prefer one over the other.

After discussions, we have come to the conclusion that the best way forward is to support that extensions can define extensionKind as an array. e.g.

• extensionKind: ["ui", "workspace"] would define that an extension can run on both sides, with a preference to running on the ui side if possible (if it is installed on the ui side)
• extensionKind: "ui" would define that an extension can only run on the ui side.

Since we need to first have one stable release out before extensions can adopt the new array format, we will maintain for a brief period that extensionKind: "ui" is treated as extensionKind: ["ui", "workspace"] in order to get compatibility and give enough time for extensions to adopt.

FYI @egamma @sandy081

[alexdima]

Implemented via c6f4d5c