pVACseq should handle frameshift that fall "within the region of the splice site"

Currently, the VEP Frameshift plugin does not process any variants that have a "splice" as one of the consequences. However, there is one special case that should be processable. If a variant is both a frameshift and a "splice_region_variant" and the variant is contained within the 1-3 bases of the exon, we should be able to treat this as any other frameshift.

To add support for this, we would need to update the Frameshift plugin to process "splice_region_variant" consequences (but still skip all other splice_x consequences). pVACseq would then need to check whether the frameshift variant falls within the 1-3 bases of the exon and only if it does, process it further.

Locate the VEP Frameshift plugin and pVACseq frameshift-processing entry points, then read their existing handling of splice consequences. Confirm the special case for variants marked both frameshift and splice_region_variant within 1–3 exon bases, while other splice_x consequences remain skipped. Add or update focused tests covering both the accepted case and excluded cases.