postCSS: Improve validation of option 'config'

[deining]

This PR follows up on #10846. It adds validation for the config option: if the value given for config is not a directory, a warning is emitted.

With this patch applied:

git clone --single-branch -b hugo-github-issue-10846 https://github.com/jmooring/hugo-testing hugo-github-issue-10846
cd hugo-github-issue-10846
npm ci
hugo_0.115.dev
Start building sites …
hugo v0.115.0-DEV-8ff4385708a6c19e6f7fb2cb9394eb0cb9c3f2fa windows/amd64 BuildDate=2023-06-28T14:51:41Z

WARN  postcss config "foo/bar.config.js" must be a directory

                   | EN
-------------------+-----
  Pages            |  1
  Paginator pages  |  0
  Non-page files   |  0
  Static files     |  0
  Processed images |  0
  Aliases          |  0
  Sitemaps         |  0
  Cleaned          |  0

Total in 3217 ms

Note the line in the middle emitting a warning.