Add ST_SharedPaths

ST_SharedPaths returns the linework shared between two (multi)linestrings or polygon boundaries, as a GEOMETRYCOLLECTION holding the same direction and opposite direction shared paths. PostGIS has it and GEOS provides `GEOSSharedPaths_r`, so **it is a near clone of the existing binary geometry to geometry binds such as ST_Difference and ST_Intersection**.

| Function | Returns |
|---|---|
| `ST_SharedPaths(geom1, geom2)` | GEOMETRY, a GEOMETRYCOLLECTION of [forward MULTILINESTRING, backward MULTILINESTRING] |

Implementer tips
- `GEOSSharedPaths_r` always returns a 2 member collection, the first holding edges shared in the same direction, the second the opposite. Empty members are still present, so the shape is fixed.
- Clean wrapper, add a `get_shared_paths` next to `get_difference` and `get_intersection` and register a struct in the same shape.
- PostGIS reference, `ST_SharedPaths` in `lwgeom_geos.c`.
- It is also the building block under the ArcGIS autoComplete editing operation, so useful beyond parity.

Sharing this as research in case it helps whoever picks it up.