Fixes#13705
With this PR, if the editor detects a disallowed heading in
`HeadingFeature`, it automatically converts it to the lowest allowed
heading.
I've also verified that disallowed headings aren't introduced when
pasting from the clipboard if the HeadingFeature isn't registered at
all. The reason this works is because the LexicalEditor doesn't have the
HeadingNode in that case.