notebooks: add initial kernel/renderer constraints

See https://github.com/microsoft/vscode/issues/119899

Backwards compatible, initially. The implementation should be
pretty unsurprising. Some churn from data wiring.

This does the bulk of the work. The only remaining item is caching
the last renderer used for notebooks in the workspace to avoid running
selection again if the user reopens/reloads the window.

src/vs/workbench/api/common/extHostNotebook.ts

@@ -124,7 +124,20 @@ export class ExtHostNotebookKernelProviderAdapter extends Disposable {
 				description: kernel.description,
 				detail: kernel.detail,
 				isPreferred: kernel.isPreferred,
-				preloads: kernel.preloads,
+				preloads: kernel.preloads?.map(preload => {
+					// todo@connor4312: back compat on 2020-04-12, remove after transition
+					if (URI.isUri(preload)) {
+						preload = { uri: preload, provides: [] };
+					}
+
+					return {
+						uri: preload.uri, provides: typeof preload.provides === 'string'
+							? [preload.provides]
+							: preload.provides === undefined
+								? []
+								: preload.provides
+					};
+				}),
 				supportedLanguages: kernel.supportedLanguages,
 				implementsInterrupt: !!kernel.interrupt
 			};