Filter the nesting level of items in the navigation block

[draganescu]

Draft

We're currently hardcoding the nesting level of links in the navigation block to 5. I believe we should allow people to change this value.

[talldan]

This might be a silly idea, but what if it were a prop of the Edit component so that it could be filtered using the existing BlockEdit filter. e.g.:

NavigationLinkEdit( { maxNestingLevel = 5, // ... } ) {

[draganescu]

That's actually not a bad idea at all. We reuse a filter why add another one.