Remove automatic camel-casing of CSS custom property names

[necolas]

Describe the issue

The original StyleX shim added camel-casing of CSS custom property names to ease internal adoption of native theme definitions that map to web theme definitions. However, custom properties are case sensitive, and we have now implemented the StyleX theming APIs. So we should remove this inconsistency and allow variable names to be case sensitive.

Expected behavior

Custom property names are case sensitive and do not get automatically camel cased.

Steps to reproduce

N/A

Test case

No response

Additional comments

No response