Ensure to transpile the PostCSS Nesting plugin (tailwindcss/nesting) (#7080)

* ensure that we compile the postcss nesting plugin

* re-add optional chaining

This will allow us to be 100% sure that we can safely call
hasOwnProperty in case we get some very strange objects.

This will now also be compiled/transpiled by esbuild.

* import the internal postcss nesting plugin

This allows us to work on it without re-compiling while running tests.
Just like we do with all other code.

* update changelog

src/postcss-plugins/nesting/plugin.js

@@ -0,0 +1,44 @@
+import postcss from 'postcss'
+import postcssNested from 'postcss-nested'
+
+export function nesting(opts = postcssNested) {
+  return (root, result) => {
+    root.walkAtRules('screen', (rule) => {
+      rule.name = 'media'
+      rule.params = `screen(${rule.params})`
+    })
+
+    root.walkAtRules('apply', (rule) => {
+      rule.before(postcss.decl({ prop: '__apply', value: rule.params, source: rule.source }))
+      rule.remove()
+    })
+
+    let plugin = (() => {
+      if (
+        typeof opts === 'function' ||
+        (typeof opts === 'object' && opts?.hasOwnProperty?.('postcssPlugin'))
+      ) {
+        return opts
+      }
+
+      if (typeof opts === 'string') {
+        return require(opts)
+      }
+
+      if (Object.keys(opts).length <= 0) {
+        return postcssNested
+      }
+
+      throw new Error('tailwindcss/nesting should be loaded with a nesting plugin.')
+    })()
+
+    postcss([plugin]).process(root, result.opts).sync()
+
+    root.walkDecls('__apply', (decl) => {
+      decl.before(postcss.atRule({ name: 'apply', params: decl.value, source: decl.source }))
+      decl.remove()
+    })
+
+    return root
+  }
+}