jsx: precompile converts props to lowercase

[r-thomson]

Version: Deno 1.45.5

When using JSX with the precompile mode, props are converted to lowercase. This is particularly problematic if you’re working with SVGs, which has case-sensitive attributes.

To test this, I set jsxImportSource to a file that replaces all the JSX functions with console.log. Given the following JSX:

<div someProp="value" />;

With "jsx": "react-jsx":

div { someProp: "value" }

With "jsx": “precompile"

[ '<div someprop="value"></div>' ]

[r-thomson]

From checking into the code, it looks like this function is responsible for lowercasing the attribute names, so this behavior seems to be intentional.

https://github.com/denoland/deno_ast/blob/e7027f4f7935016b706671ab1ee239c40a55c75e/src/transpiling/jsx_precompile.rs#L89

However, this behavior still absolutely breaks some case-sensitive attributes, like viewBox, so I think it would make sense to rethink this.

[marvinhagemeister]

Reopening until the updated crate is merged here.