Added a page decorator and Jinja.page + run sync routes in threadpool

[volfpeter]

Changes

• Added a page() decorator for unconditionally rendering HTML (useful when a route needs to serve HTML for non-HTMX requests).
• Added a Jinja.page() decorator, the Jinja2 implementation of the generic page() decorator.
• Sync routes are now executed in a threadpool to avoid blocking the asyncio event loop.
• Added Jinja.hx() as the primary decorator for HTMX-serving routes for overall consistency in the library. At the same time, Jinja.__call__() and Jinja.template() are now deprecated and will be removed in the future.
• JinjaContext.unpack_result() can now handle None as well by converting it into an empty Jinja rendering context.
• Typing improvements.
• Renamed HTMXRenderer to HTMLRenderer. HTMXRenderer is still importable as a deprecated alias of HTMLRenderer.
• Updated the existing Jinja example to showcase the updated Jinja class.
• Added a fully-working (although rather basic) custom rendering example.
• Updated the documentation.

Internals

A lot of internal code has been moved. If, for some reason you imported something directly from a module instead of the package, you will unfortunately need to update your imports. New package layout:

• fasthx.core_decorators: the hx() and page() decorators can now be found here.
• Jinja-related code has been moved from fasthx.main to fasthx.jinja.
• get_hx_request() and DependsHXRequest are now in fasthx.dependencies.
• All utility types have been moved from fasthx.main to fasthx.typing.
• The internal _append_to_signature() method was moved from fasthx.main to fasthx.utils and renamed to append_to_signature. This method is still for internal use only.

Notes

Tentatively bumped version to 0.2402.2, although putting so many (and important) changes in a patch release is not that user-friendly. I'm considering moving to semver and 1.0 at this point, or alternatively to 0.2403.0.

[volfpeter]

Leaving the PR here for a day or two, to leave some time for questions or comments.