feat: Allow power armor weapon racks to sheath spears

[karxi]

Checklist

Required

• [x] 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.
• [x] I have committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.

Purpose of change

The power armor weapon rack has the maximum volume required to sheath a lucerne hammer (and is the only sheath with this distinction), and seems to be set up to allow storing nearly any melee weapon, but a missing flag prevents this.

Describe the solution

Add "SHEATH_SPEAR" to power_armor_back_hardpoint's "use_action"/"flags" field.

Testing

Inserted and removed a lucerne hammer and survivor naginata without issue.