Fix native ESM config loading in v3 (#18938)

Unfortunately for backwards compatibility purposes (with `loadConfig` at
least) we can't switch things to use `import(…)` because there's baked
in knowledge that the config is loaded synchronously for v3.

This PR does two things:
- Defers to `require(…)` which allows newer versions that support
`require(esm)` to work natively. This works around the need to switch to
`import(…)` for those versions.
- Allows newer versions of `postcss-load-config` enabling better
ESM+TypeScript support for PostCSS configs in the CLI.

We support v4, v5, and v6 of `postcss-load-config` simultaneously so any
of those versions should work. I've verified that newer node versions
seem to install v6 while earlier ones like Node v14 install v4 of
`postcss-load-config`. So this should be a backwards compatible change.

- [x] needs tests for `import.meta.resolve(…)`
- [x] needs tests for ESM postcss configs

Fixes #14152
Fixes #14423

package.json

@@ -72,7 +72,7 @@
     "fast-glob": "^3.3.2",
     "glob-parent": "^6.0.2",
     "is-glob": "^4.0.3",
-    "jiti": "^1.21.6",
+    "jiti": "^1.21.7",
     "lilconfig": "^3.1.3",
     "micromatch": "^4.0.8",
     "normalize-path": "^3.0.0",
@@ -81,7 +81,7 @@
     "postcss": "^8.4.47",
     "postcss-import": "^15.1.0",
     "postcss-js": "^4.0.1",
-    "postcss-load-config": "^4.0.2",
+    "postcss-load-config": "^4.0.2 || ^5.0 || ^6.0",
     "postcss-nested": "^6.2.0",
     "postcss-selector-parser": "^6.1.2",
     "resolve": "^1.22.8",