update to new apis

[brettfo]

This is a big one.

The VS Code notebook APIs have taken a large change that's mostly incompatible with the current Stable release. Because of that a bunch of common code had to be moved into the stable/src and insiders/src directories and other helpers rearranged as necessary.

These changes aren't meant to be used in Stable currently, only in Insiders so we can unblock our users. The differences between the two will be reconciled before the next Stable release which will then bring both back into close alignment.

There are some bugs yet to be filed against VS Code once this has been merged, specifically:

• Notebook controller webview not getting postMessage calls via onDidReceiveMessage(). Because of this bug, JavaScript cells don't behave as expected. (Webview post messages are not posted to the notebook controller microsoft/vscode#121895)
• onDidChangeNotebookAssociation should fire even on initial notebook open (onDidChangeNotebookAssociation not fired on initial notebook open microsoft/vscode#121904).
• cannot set cell language using mouse on the cell itself, only via command palette (Cannot set cell language via ui microsoft/vscode#121927)

[colombod]

microsoft/vscode#121895 was closed and latest insiders addressed the webview ipc issue

[colombod]

microsoft/vscode#121904 tracks the issue with onDidChangeNotebookAssociation not fired

[colombod]

Can't select cell language with mouse gesture, it flashes only, using the command to select the cell language works and also triggers the onDidChangeNotebookAssociation