move enum json & yaml generation in the related formatter

[AlbertoBarba]

• Move Enum Marshal/Unmarshal generation into formatters
• Add missing Enum Marshal/Unmarshal generation for YAML format
• Make formatters stateless and instantiate them only once
• Split formatters in multiple files

[codecov[bot]]

Codecov Report

Attention: 34 lines in your changes are missing coverage. Please review.

Comparison is base (e7be9e2) 77.99% compared to head (94f624e) 77.38%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #134 +/- ## ========================================== - Coverage 77.99% 77.38% -0.62% ========================================== Files 17 18 +1 Lines 1718 1795 +77 ========================================== + Hits 1340 1389 +49 - Misses 293 320 +27 - Partials 85 86 +1 ``` | [Files](https://app.codecov.io/gh/omissis/go-jsonschema/pull/134?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Claudio+Beatrice) | Coverage Δ | | |---|---|---| | [pkg/generator/generate.go](https://app.codecov.io/gh/omissis/go-jsonschema/pull/134?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Claudio+Beatrice#diff-cGtnL2dlbmVyYXRvci9nZW5lcmF0ZS5nbw==) | `74.59% <100.00%> (-0.94%)` | :arrow_down: | | [pkg/generator/json\_formatter.go](https://app.codecov.io/gh/omissis/go-jsonschema/pull/134?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Claudio+Beatrice#diff-cGtnL2dlbmVyYXRvci9qc29uX2Zvcm1hdHRlci5nbw==) | `100.00% <100.00%> (ø)` | | | [pkg/generator/yaml\_formatter.go](https://app.codecov.io/gh/omissis/go-jsonschema/pull/134?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Claudio+Beatrice#diff-cGtnL2dlbmVyYXRvci95YW1sX2Zvcm1hdHRlci5nbw==) | `75.00% <75.00%> (ø)` | | | [tests/data/extraImports/gopkgYAMLv3/gopkgYAMLv3.go](https://app.codecov.io/gh/omissis/go-jsonschema/pull/134?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Claudio+Beatrice#diff-dGVzdHMvZGF0YS9leHRyYUltcG9ydHMvZ29wa2dZQU1MdjMvZ29wa2dZQU1MdjMuZ28=) | `35.00% <40.00%> (+15.00%)` | :arrow_up: |

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