feat: declare acquireVsCodeApi on the global namespace

[RedGlow]

When using the bundled version of the Messenger class in vscode-messenger-webview (the one built in the lib directory), the code that gets imported is something like this:

vscode-api.js:

Object.defineProperty(exports, "__esModule", { value: true });

(makes sense it's empty, since declare and interface don't produce javascript code)

messenger.js:

const vscode_api_1 = require("./vscode-api");
...
        this.vscode = vscode !== null && vscode !== void 0 ? vscode : (0, vscode_api_1.acquireVsCodeApi)();

(this is wrong: acquireVsCodeApi is defined on globalThis/window, not in the module!)

By moving the declaration of acquireVsCodeApi on the global namespace, the bundled code is now:

        this.vscode = vscode !== null && vscode !== void 0 ? vscode : acquireVsCodeApi();

(notice that there's no longer a reference to vscode_api_1 or ./vscode-api)

[dhuebner]

Thanks for contributing!