feat: add relation support

[cuebit]

This PR adds relation support.

Since all relations are essentially "sub queries" they do not inherit top level query chain modifiers. This is particularly problematic when using the onlyTrashed modifier, which will never resolve soft deleted relations, because the plugins default behaviour is to ignore soft deleted models and there is currently no informing relation queries to inherit the type of filtering.

Type of PR:

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

Breaking changes:

• No
• Yes

[codecov-io]

Codecov Report

Merging #8 into master will not change coverage by %.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master        #8   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            8         8           
  Lines          128       143   +15     
  Branches        13        16    +3     
=========================================
+ Hits           128       143   +15     

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

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[kiaking]

Very nice! Yea maybe we should be doing something with the Vuex ORM core limitation 🤔 Do you think the PR is ready to be merged?

[cuebit]

@kiaking yea, I realised when working with TS I’m finding it difficult to tap into certain processes where it would otherwise be straightforward with JS. Currently there is no way for TS to get a hold of relations other than using the query chain, but users may add criteria that may also affect the intended behaviour of the plugin therefore it’s a trivial problem.

Regardless, I’m always keen to find the least expensive solution and this is the best I could come up after hours of alternatives. If the ecosystem is to flourish I think the core api will need to be a little more lenient. But that’s a discussion for another day.

This PR is ready for merge and release. 👍

[kiaking]

Yea, you're right. I think we could discuss more on core design 👍

Thanks for this PR! Let's make a release on this 💪