Adopt Fetch API's Headers interface

[scriptogre]

Replace the request header plain object with a native Headers instance.

Updates request building, hx-config, hx-headers, htmx.ajax(), and bundled extensions to use the standard Headers API.

HCON.merge(...) now knows how to merge into Headers, so existing object-style config values still work:

<div hx-config="headers.X-Custom: value"></div>

Added an hx-compat-htmx-4-beta auto-registered extension that keeps ctx.request.headers[name] working and warns with the new syntax:

[htmx] Deprecated: replace ctx.request.headers[name] with Headers methods 
  old: ctx.request.headers[name]
  new: ctx.request.headers.get(name), .set(name, value), or .delete(name)
  docs: https://developer.mozilla.org/en-US/docs/Web/API/Headers

The extension might seem a bit overengineered for what it does now, but it was done to cleanly accommodate some other stuff I'll propose in a PR soon.