Refactor to use PostCSS Container type for more flexibility

[ybiquitous]

Which issue, if any, is this issue related to?

Fixes the type error on #5689

Is there anything in the PR that needs further explanation?

Instead of import('postcss').Rule | import('postcss').AtRule, this refactoring uses the import('postcss').Container type.
This type includes also import('postcss').Root so has more flexibility.

See the type definitions of PostCSS below:

• Container - https://github.com/postcss/postcss/blob/77c2e73/lib/container.d.ts#L23-L30
• AtRule - https://github.com/postcss/postcss/blob/77c2e73/lib/at-rule.d.ts#L73
• Rule - https://github.com/postcss/postcss/blob/77c2e73/lib/rule.d.ts#L66
• Root - https://github.com/postcss/postcss/blob/77c2e73/lib/root.d.ts#L51

[jeddy3]

Cleaner and more flexible, thanks!