This PR adds support for Maud templates in Rust.
We already had some pre-processing for Rust but for Leptos `class:`
syntax. This PR now added a dedicated Rust pre-processor that handles
Leptos and Maud syntax.
We only start pre-processing Maud templates if the Rust file includes
the `html!` macro.
## Test plan
Looking at the extractor, you can see that we now do extract the proper
classes in Maud templates:
<img width="1076" height="1856" alt="image"
src="https://github.com/user-attachments/assets/e649e1de-289e-466f-8fab-44a938a47dd5"
/>
Fixes: #18984
85575a41f4