Add react-strict-dom/postcss-plugin based on @stylexjs/postcss-plugin

[javascripter]

Why

#281 (comment)

This PR upstreams postcss-react-strict-dom to the official repo to make it easier for maintainers to make changes.

I added RSD compatibility to @stylexjs/postcss-plugin in this PR, so this adds react-strict-dom/postcss-plugin that re-exports the StyleX PostCSS Plugin with the required configuration for RSD consumers.

How

• Added PostCSS Plugin to packages/react-strict-dom/postcss/plugin.js
• Export the plugin as react-strict-dom/postcss-plugin in package.json
• Updated apps/examples to use the official plugin and removed postcss-react-strict-dom dependency
• Updated docs to use the official plugin

Test Plan

• Run cd apps/example && npm run dev:web both from the original repo and this branch and compare side-by-side to make sure output matches

Left is this branch, right is original

[necolas]

Suggested change

	useCSSLayers: false,
	useCSSLayers: true,

[javascripter]

It seems Expo Web doesn't support CSS Layers. If we enable useCSS Layers and run npm run dev:web, it shows the below warning.
I disabled CSS layers for now:
b455d43

| Warning: Unknown at rule: @property (src/app/stylex.css:16:10):
|   14 |
|   15 |  @layer priority1{
| > 16 | @property --backgroundColor { syntax: "*"; inherits: false;}
|      |          ^
|   17 | @property --opacity { syntax: "*"; inherits: false;}
|   18 | @property --transform { syntax: "*"; inherits: false;}
|   19 | :root, .xxw3bu5{--primary100-x1ud21w4:black;--primary200-x19lv8z8:white;}
| Warning: Unknown at rule: @property (src/app/stylex.css:17:10):
|   15 |  @layer priority1{
|   16 | @property --backgroundColor { syntax: "*"; inherits: false;}
| > 17 | @property --opacity { syntax: "*"; inherits: false;}

[necolas]

Looks like it might be a StyleX bug. Don't think you're allowed to nest @property inside @layer in CSS. (Chrome treats this as valid AFAICT.)

Maybe it's a CSS validator in Expo that doesn't allow it within layers?

[necolas]

#311 updates StyleX to 0.13

[javascripter]

Rebased from main and updated to use useCSSLayers: true. Waiting for facebook/stylex#1071 to land so CSS layer warnings will disappear

[necolas]

This PR aims to avoid the Expo warnings facebook/stylex#1087