Move HighlightSpacing to ratatui-core or new ratatui-common-widget-components

Problem

HighlightSpacing is used by table and list but is currently located with the table. (results in odd imports for List: use crate::table::HighlightSpacing;)

More external widgets might want to use it which shouldn't need to depend on the whole ratatui-widgets.

Solution

Move the HighlightSpacing enum to ratatui-core or some ratatui-common-widget-components. (When thinking about the latter, Scrollbar or Block would be also good candidates for such an in-between crate. Or even better: split every widget into its own crate so ratatui-list-widget and ratatui-table-widget exist on their own, removing the ratatui-widget crate. Should probably increase compile time on multicore systems and allows development cycles independent of each other. Probably worth its own issue, shouldn't be discussed here.)

Alternatives

Additional context