add support for n.* (out-of-bounds) transcript variants

[reece]

With #437, transcript coordinates may now be outside of the transcript bounds.

Absolute numbers (e.g., n.-5 and n.500 for a tx w/length<500) are now supported when strict_bounds=False).

Currently, hgvs does NOT support * coordinates, such as *.5. This issue proposes such support. This requires:

• supporting * in the grammar
• transforming numeric coordinates to intervals with datum=SEQ_END

This change also incurs some risk because it's likely that hgvs code and client code expect datum=SEQ_START for n. variation.