[FEATURE] - HFB support for iMOD5 models with linearrings

@FransRoelofsen ran into this for MORIA and after discussion with @Huite:

iMOD5 supported providing HFBs as linearrings as well in the GEN file. There are models which rely on GEN files containing both linestrings as wells as linearrings in a single file. We now automatically convert linearrings in GEN files to polygons. I think this is what most users expect. In addition, MODFLOW6 HFBs now only expect linestrings. Sometimes users may want to provide a linearring which could be a building pit or something. I think this should be a linearring, as polygons would lead to headaches: What to do with polygons with interior rings?

We should therefore do the following to support these:

• In GEN file reader: Split up linestrings and polygons in different lists, separately convert to shapely objects, then merge together into single GeoDataframe
• in iMOD5 HFB converter: Convert polygon to linearring
• in MODFLOW6 : Add support for providing HFBs as linearrings, to simulate things like building pits.

Start with the GEN file reader, the iMOD5 HFB converter, and MODFLOW6 HFB handling described in the issue. Trace how linestrings, polygons, and linearrings are currently converted, then verify that mixed GEN geometries are preserved, iMOD5 polygons become linearrings, and MODFLOW6 accepts linearrings for building pits.