slintpad/vscode: Get rid of the shared files

Theyre are no more shared files, so move the files
into vscode and set up a syntax check over there.

That was not possible before as the typescript
compiler would error out on the shared files being
outside its root directory.

editors/vscode/src/lsp_commands.ts

@@ -0,0 +1,26 @@
+// Copyright © SixtyFPS GmbH <info@slint.dev>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+import { URI as LspURI } from "vscode-languageserver-types";
+import * as vscode from "vscode";
+
+// Use the auto-registered VSCode command for the custom executables offered
+// by our language server.
+//
+// Talking to the server directly like this:
+//
+// ```typescript
+//     return await client.sendRequest(ExecuteCommandRequest.type, {
+//         command: "slint/showPreview",
+//         arguments: [url, component],
+//     } as ExecuteCommandParams);
+// ```
+//
+// has the side effect of going around our middleware.
+
+export async function showPreview(
+    url: LspURI,
+    component: string,
+): Promise<unknown> {
+    return vscode.commands.executeCommand("slint/showPreview", url, component);
+}