make vim strict ui extension

[sandy081]

Hi, this is developer from VS Code. As we are rolling out web extensions we are trying to fine tune and simplify the concepts around web extensions. As a result, we are no longer supporting web as an extension kind to identify as a web extension. Instead we infer it from other properties - See https://github.com/microsoft/vscode-docs/blob/vnext/api/extension-guides/web-extensions.md#web-extension-enablement for more details. With latest vsce, it will validate extension kind for appropriate values which are ui, workspace. Since this extension has web extension kind, this PR fixes it properly.

It seems vim aways wants to run close to UI, so it is recommended to mark it as strict ui extension so that it will run close to UI in any environment.

Hence changed vim extension kind to be strict ui.

[sandy081]

@rebornix FYI

[rebornix]

@sandy081 thanks!