Add transcript length to all transcript objects

Currently, transcript length is calculated by iterating over the exons, computing the difference between each exon’s start and end positions, and summing those values. It would be much easier if this was recalculated and stored for each transcript.

This could be split into two variables: one representing the full transcript length based on exons, including UTRs, which could be `transcript.length`, and another representing the CDS-only length, which could be stored as `transcript.length_cds`. Curious for the dev teams thoughts!

Requested by @gkaur