Don't allow continuous polygons to collapse into dust

[e-n-f]

With the --no-simplification-of-shared-nodes option, shard gaps could still be introduced between polygons through interaction with the collapse of tiny polygons into polygon dust. This PR prevents polygons that have been determined to share a border with another polygon from collapsing into dust.

Affected test

tests/ne_110m_admin_0_countries/out/-D8_-d8_-z1_-yname_--no-simplification-of-shared-nodes.json:

Before:

After:

[ChrisLoer]

Erica and I have talked this through, with two things worth noting:

• Because continuous polygons can't dustify anymore, that means there will be more cases where tippecanoe gets pushed into coalescing continuous polygons, which is potentially more expensive. Erica plans to time the Texas census blocks case before and after the change to get an idea for how much impact that might make.
• Any contact of a multipolygon with another polygon makes the whole multi-polygon non-dustable, but this seems unnecessary in most cases and can have unintuitive results (the case that got us talking about it: why are Papua New Guinea's islands preserved but the next door Solomon Islands discarded). But this goes in the "further refinements" bucket, not a blocker at all.

[ChrisLoer]

For manual testing, my plan is to test within Felt on the matched Felt PR, rather than testing directly with tippecanoe.