Add support for notebook types in DocumentSelector

[jrieken]

Throughout the API the DocumentSelector is used to "bind" features like completions or code navigations against certain documents. However, it doesn't allows to narrow down on documents that are part of a notebook, extensions want to say python-language but only in jupyter-notebooks.

/cc @dbaeumer who brought this up for the notebook related LSP work

[jrieken]

This is API delta/addition which complements the existing DocumentFilter:

vscode/src/vscode-dts/vscode.proposed.notebookDocumentSelector.d.ts

Lines 10 to 16 in 609eea3

	export interface DocumentFilter {
	/**
	* The {@link NotebookDocument.notebookType type} of a notebook, like `jupyter`
	*/
	readonly notebookType?: string;
	}

The presence of notebookType implicitly means that the scheme is vscode-notebook-cell (meaning it must be that value or undefined).

---

Also, the idea of also allowing to filter on notebook kernels/controllers didn't stick. A controller is a private object, meaning a controller that adds language features knows the current notebook and can filter on that, e.g in its onDidChangeSelectedNotebooks-handler it can create a filter targeting the notebook type and its path, like so

// controller-specific language feature
 myKernel.onDidChangeSelectedNotebooks(e => {

    vscode.languages.registerTypeDefinitionProvider({
        language: 'plaintext',
        pattern: e.notebook.uri.path,
        notebookType: e.notebook.notebookType
    }, new class implements vscode.TypeDefinitionProvider {
        provideTypeDefinition(document: vscode.TextDocument, position: vscode.Position, token: vscode.CancellationToken) {

            return new vscode.Location(document.uri, new vscode.Position(0, 0));
        }
    })
 })

[jrieken]

When notebookType is set, change the meaning of scheme and pattern to be checked against the notebook values, not the cell document

[jrieken]

@dbaeumer please verify that this is stable API now