[webchannel] Add interactive TS web demo

[sampajano]

Summary

This PR adds a simple, interactive TypeScript Web Demo for the compiled WebChannel client bundle.

It provides an E2E playground allowing developers to test connectivity against the live Google sandbox endpoints (/staging/channel/generator and /staging/channel/echo).

Key Features Supported:

• Endpoint Dropdown Selector: Easily switch between Generator Server (for streaming push verification) and Echo Server (for interactive latency measurement).
• Option Toggles: Configure key properties (sendRawJson, detectBufferingProxy, forceLongPolling, fastHandshake) during connection handshake.
• UX Controls: Fades out buttons based on connection status, keeps input editable, and includes a Clear Logs option.

Note

We will revert the compiled WebChannel JS bundle (webchannel_blob) soon, once the core webchannel library is migrated to TypeScript.

[wenbozhu]

Thanks!