Add TestVsCode harness

[manzt]

Adds a test harness (TestVsCode) that simulates the VSCode environment. This harness provides a controlled context where VsCode state can be manipulated and observed in our tests.

We can now programmatically create scenarios (opening notebooks, executing cells, triggering commands) and verify the resulting system state.

const vscode = yield* TestVsCode.make();

yield* vscode.addNotebookDocument(/* ... */)

yield* Effect.provide(
  Effect.gen(function* () {
    const {commands}  = yield* VsCode;
    yield* commands.registerCommand("myCommand", Effect.void);
  }),
  vscode.layer, // <- Provides the VsCode service
);

expect(yield* vscode.snapshot()).toMatchInlineSnapshot(`{
  commands: ["myCommand"],
  serializers: [],
  controllers: [],
}`)