Generate the dioxus_html/src/elements.rs with codegen

[rambip]

For now, it is just initial work.

Motivation:

• easier to read and to contribute, in particular for people who would like to use dioxus with other elements.
• if it works, dioxus-cli would not need to be compiled using the exact same version of dioxus_elements. The idea is that the cli could look at the dependencies, find what element library is used, get the elements.toml, and use it to do hot-reloading.

Missing:

• syntax to specify the same attributes for a lot of elements
• what name to give to the a element in the svg namespace.

[jkelleyrtp]

Hi there, just closing this since it's been up for a while. Please feel free to re-open it or make a new PR if anything updates here!