feat: Add portal animation

[thedyze]

Checklist

Required

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax so it can be closed automatically.
• I have committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.

Optional

• I have documented the changes in the appropriate location in the doc/ folder.

Purpose of change

Make shimmering portals shimmer.

Describe the solution

Change to an animated sprite for "tr_portal", create frames for this and add to the external_tileset.

Describe alternatives you've considered

Testing

Additional context

Also added documentation of my previous PR for the blood sprites.