feat: add relations to plugin scope

[cuebit]

Type of PR:

• Bugfix
• Feature
• Refactor
• Code style update
• Build-related changes
• Test
• Documentation
• Other, please describe:

Breaking changes:

• No
• Yes

Details

Adds BelongsTo, HasManyBy and MoprhOne to plugin scope.

[codecov]

Codecov Report

Merging #103 (418bd45) into master (8d5245a) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #103   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           38        38           
  Lines         1153      1156    +3     
  Branches       185       185           
=========================================
+ Hits          1153      1156    +3     

Impacted Files	Coverage Δ
src/plugin/Plugin.ts	100.00% <100.00%> (ø)

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[kiaking]

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