Add JSON output type

[caitlinelfring]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
Feature

What is the current behavior? (You can also link to an open issue here)
No JSON printer

What is the new behavior (if this is a feature change)?
Outputs file results as a json string. This will be very helpful for IDE integrations

Note that this will not be an array of FileResults, but will be separate JSON strings for each file result, since the results are written to the console asynchronously.

Does this PR introduce a breaking change? (What changes might users need to make due to this PR?)
No

Other information:

[codecov]

Codecov Report

Merging #26 into main will increase coverage by 0.87%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   93.80%   94.67%   +0.87%     
==========================================
  Files          18       19       +1     
  Lines         436      451      +15     
==========================================
+ Hits          409      427      +18     
+ Misses         16       13       -3     
  Partials       11       11              

Impacted Files	Coverage Δ
pkg/printer/json.go	100.00% <100.00%> (ø)
pkg/printer/printer.go	100.00% <100.00%> (ø)
pkg/result/lineresult.go	100.00% <100.00%> (+7.89%)	⬆️
pkg/rule/severity.go	92.85% <100.00%> (+0.26%)	⬆️

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