URL Source: pre/post request scripts

- Optional JS/WASM snippet to transform the request before sending or the response before displaying

Part of #68.

Read parent issue #68 first, then inspect the URL Source entry point mentioned here. Determine how optional JavaScript/WASM snippets should transform requests before sending and responses before display, and define completion around both transformations working without scripts remaining optional.

由索引模型根據 Issue 內容生成。