Declarative partial updates

Chrome 148 introduces two new web platform APIs under the Declarative Partial Updates initiative. The first enables out-of-order HTML streaming using processing instructions (`<?marker>`, `<?start>`, `<?end>`) as placeholders and `<template for>` elements to fill them, enabling island architecture and deferred content delivery without JavaScript. The second is a revamped suite of static and streaming JavaScript HTML insertion methods (`setHTML`, `streamHTML`, `appendHTML`, `streamAppendHTML`, etc.) with consistent naming, sanitizer support, and Streams API integration. Both APIs have polyfills available on npm. Together they allow SPA-style partial page updates driven by streamed HTML, reducing reliance on heavy JavaScript frameworks for dynamic content.