[css-regions-1] `flow-into` and `flow-from` should exclude all CSS-wide keywords

[cdoublev]

I think <ident> should be <custom-ident> in both property value definitions, and it should exclude none in the value definition of flow-from

Name: flow-into
Value: none | <ident> [element | content]?

The values none, inherit, default, auto and initial are invalid flow names.

Name: flow-from
Value: <ident> | none

[cdoublev]

Fixed by #12445.