feat: support spell links in /phd spell command (#23)

[Xerrion]

Description

Adds support for parsing WoW spell links in the /phd spell command so users can shift-click spells from their spellbook instead of typing the exact name.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Refactor (non-breaking change that improves code quality)
• Documentation update
• Breaking change (fix or feature that would cause existing functionality to change)

Related Issues

Closes #23

Testing

• Luacheck passes (luacheck .)
• Tested in-game manually
• WoW version(s) tested on: TBC Anniversary

Checklist

• My code follows the project's code style (4-space indent, 120 char lines)
• I have tested my changes in-game
• Luacheck reports no warnings
• My commits follow conventional commit format