"DX" chapter - add suggestions for Rust compilation time improvements; add tw & emmet support instructions as well

[diversable]

Compile-time decreases lead to a major "quality of life" improvement - advice in the DX chapter should be added to help improve Rust compile times in Leptos apps:
See: https://benw.is/posts/how-i-improved-my-rust-compile-times-by-seventy-five-percent

Also, since tailwind and emmet support are not natively set up for Rust, add instructions on how to add support for tailwind and emmet to VSCode (and other editors, if possible).

See the instructions for tw & emmet here:
https://github.com/leptos-rs/leptos/blob/main/examples/tailwind_csr/README.md