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

.github/workflows/ci-stable.yml

@@ -27,7 +27,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [14, 18]
+        node-version: [14, 18, 20, 22, 24]
 
     steps:
       - uses: actions/checkout@v3

.github/workflows/ci.yml

@@ -24,7 +24,7 @@ jobs:
 
     strategy:
       matrix:
-        node-version: [14, 18]
+        node-version: [14, 18, 20, 22, 24]
 
     steps:
       - uses: actions/checkout@v3

.github/workflows/integration-tests.yml

@@ -31,7 +31,7 @@ jobs:
           - vite
           - webpack-4
           - webpack-5
-        node-version: [18]
+        node-version: [18, 20]
       fail-fast: false
 
     steps: