[Magic] Add Shield Barrier trait functionality

[ampitere]

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• [x] I understand that if I do not agree to the following points by completing the checkboxes my PR will be ignored.
• [x] I understand I should leave resolving conversations to the LandSandBoat team so that reviewers won't miss what was said.
• [x] I have read and understood the Contributing Guide and the Code of Conduct.
• [x] I have tested my code and the things my code has changed since the last commit in the PR and will test after any later commits.

What does this pull request do?

Adds functionality for the PLD Shield Barrier trait as described on the wiki. Adds the defense value of the equipped shield to the power of all Protect spells cast by self.

https://www.bg-wiki.com/ffxi/Shield_Barrier

Steps to test these changes

1. !changejob 7 99
2. !getmod shield_barrier should be 1
3. !addallspells
4. !additem 15070
5. Cast Protect with shield on, bonus should apply
6. !changejob 7 69
7. !getmod shield_barrier should be 0
8. Cast Protect with shield on, bonus shouldn't apply

Protect base 20 Protect with shield 60 Protect II base 50 Protect II with shield 90 Protect III base 90 Protect III with shield 130 Protect IV base 140 Protect IV with shield 180 Protect V base 220 Protect V with shield 260