Add prettyPrintIndentation property on XMLEncoder

[MaxDesiatov]

New property can take XMLEncoder.PrettyPrintIndentation values such as .tabs(1) or .spaces(2).

Resolve #183.

[github-actions]

	Fails
🚫	Error deserializing SwiftLint JSON response (): dataCorrupted(Swift.DecodingError.Context(codingPath: [], debugDescription: "The given data was not valid JSON.", underlyingError: Optional(Error Domain=NSCocoaErrorDomain Code=3840 "The data is not in the correct format.")))

Generated by 🚫 Danger Swift against e9b2bdd

[codecov]

Codecov Report

Merging #186 into master will increase coverage by 0.19%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #186      +/-   ##
==========================================
+ Coverage   73.32%   73.52%   +0.19%     
==========================================
  Files          43       43              
  Lines        2332     2342      +10     
==========================================
+ Hits         1710     1722      +12     
+ Misses        622      620       -2     

Impacted Files	Coverage Δ
Sources/XMLCoder/Auxiliaries/XMLCoderElement.swift	96.36% <100.00%> (+1.03%)	⬆️
Sources/XMLCoder/Encoder/XMLEncoder.swift	86.01% <100.00%> (+0.40%)	⬆️

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