Add VS Code extension to view Zephyr trace files in-editor

[jbicker]

Summary
Adds a VS Code extension that opens the existing Zephelin Trace Viewer webapp inside the editor as a custom editor for Zephyr RTOS trace files (JSON).

Changes
Extension (extension/): Custom editor provider that opens *.json trace files in a webview. It reuses the built webapp from dist/ (from yarn build), rewrites asset paths to webview URIs, injects the opened file’s trace data as base64 into the webview, and sets a CSP with nonce for scripts.
Build: Extension is built with esbuild; root package.json adds build:ext and build:all so the webapp and extension can be built together.
VS Code: .vscode/ tasks and launch config for building and running the extension; .gitignore updated for extension/out, extension/node_modules, and selected .vscode files.

How to try
Run yarn build:all (or yarn build then yarn build:ext).
Run the “Launch Extension” debug config (F5).
In the new Extension Development Host, open a .json trace file; it should open with “Zephelin Trace Viewer”.

[glatosinski]

Hi, thanks for contributing - could you please fix lint issues introduced in the post message handling?

[glatosinski]

Could you please rebase and clean up git commit history so that merge commit is removed?