Feature/root class instances

[pie6k]

Now things like

@SchemaRoot()
class FooSchema {
  private bar: number = 42;

  @Query()
  foo(): number {
    return this.bar;
  }
}

are possible.

This will allow Dependency Injection at schema root level

[codecov[bot]]

Codecov Report

Merging #39 into master will decrease coverage by 0.27%. The diff coverage is 93.47%.

@@            Coverage Diff             @@
##           master      #39      +/-   ##
==========================================
- Coverage    87.7%   87.43%   -0.28%     
==========================================
  Files          60       61       +1     
  Lines        1074     1106      +32     
  Branches      189      194       +5     
==========================================
+ Hits          942      967      +25     
- Misses        131      138       +7     
  Partials        1        1

Impacted Files	Coverage Δ
src/domains/field/compiler/resolver.ts	91.95% <100%> (+0.28%)	:arrow_up:
src/domains/schema/rootFields.ts	92.3% <100%> (-0.29%)	:arrow_down:
src/domains/schema/error.ts	67.56% <100%> (-7.44%)	:arrow_down:
src/services/utils/deprecation/index.ts	100% <100%> (ø)	:arrow_up:
src/domains/schema/index.ts	86.95% <100%> (+1.95%)	:arrow_up:
src/domains/schema/compiler.ts	94.11% <100%> (-0.48%)	:arrow_down:
src/domains/field/compiler/services.ts	88.88% <50%> (ø)	:arrow_up:
src/domains/schema/services.ts	90.9% <90.9%> (ø)
... and 5 more

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