[Lua] Simplified npc_util.lua logic, converted variable naming across files

[Flibe-XI]

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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?

Tidy up variable naming convention for reward tables : 'ki' -> 'keyItem' & 'xp' -> 'exp' across many files, eliminating need for ambiguous variable handling in npc_utils.lua and roe.lua.

Steps to test these changes

Complete one of the affected quest, mission, or ROE, get appropriate rewards.