fix(types): add partial user settings

[oriori1703]

Summary

• add a recursively partial MasonUserSettings type for public setup input
• keep MasonSettings as the complete resolved internal type
• preserve nested completion and value validation

Verification

• make test
• StyLua and git diff --check
• EmmyLua 0.25.1 and LuaLS 3.19.1 with empty, top-level, and nested partial settings
• negative type-check fixtures for invalid nested values

Closes #2114