feat: add lazy relationship loading feature

[kiaking]

This PR adds lazy relationship loading feature. See docs for the API.

Type of PR:

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

Breaking changes:

• No
• Yes

[codecov]

Codecov Report

Merging #44 (d44eac0) into master (fd8314f) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #44   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           36        36           
  Lines          813       816    +3     
  Branches       126       127    +1     
=========================================
+ Hits           813       816    +3     

Impacted Files	Coverage Δ
src/model/Model.ts	100.00% <100.00%> (ø)
src/query/Query.ts	100.00% <100.00%> (ø)

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