Inline plugins (#5455)

* filter out `..Variant` plugins for the core-plugin-list

* inline all corePlugins

* move preflight css to `./src/css`

* remove individual plugins

* convert export default object to named exports

Note: Normally I would use export function ..., but since we also have
some export let xx = createUtilityPlugin in this file, it means that all
the `export function` declarations would be hoisted and therefore won't
have the correct order anymore.

To fix this, I used `export let xx = () => {}` instead of the usual
`export function xx() {}`

* drop unused `variants()` function

This was required for AOT mode.

* make a few plugins shorter

scripts/create-plugin-list.js

@@ -1,8 +1,8 @@
-import * as corePlugins from '../src/plugins'
+import * as corePlugins from '../src/corePlugins'
 import fs from 'fs'
 import path from 'path'
 
-const corePluginList = Object.keys(corePlugins)
+let corePluginList = Object.keys(corePlugins).filter((plugin) => !plugin.includes('Variants'))
 
 fs.writeFileSync(
   path.join(process.cwd(), 'src', 'corePluginList.js'),