Apply SwiftFormat on CI runs

[MaxDesiatov]

We currently have a plenty of formatting issues such as inconsistent indentation, trailing whitespaces etc in the codebase. In addition, formatting-related coding style approaches are not enforced. Here latest changes from SwiftFormat are applied and in addition it is set up to execute checks on CI.

[codecov]

Codecov Report

Merging #32 into master will not change coverage.
The diff coverage is 46.58%.

@@           Coverage Diff           @@
##           master      #32   +/-   ##
=======================================
  Coverage   51.57%   51.57%           
=======================================
  Files          29       29           
  Lines        1400     1400           
=======================================
  Hits          722      722           
  Misses        678      678

Impacted Files	Coverage Δ
Sources/XMLCoder/Box/NullBox.swift	33.33% <0%> (ø)	⬆️
Sources/XMLCoder/Auxiliaries/XMLStackParser.swift	86.53% <100%> (ø)	⬆️
Sources/XMLCoder/Box/FloatBox.swift	90% <100%> (ø)	⬆️
...urces/XMLCoder/Auxiliaries/String+Extensions.swift	100% <100%> (ø)	⬆️
Sources/XMLCoder/Encoder/XMLEncoder.swift	48.21% <100%> (ø)	⬆️
Sources/XMLCoder/Box/BoolBox.swift	86.66% <100%> (ø)	⬆️
Sources/XMLCoder/Box/DateBox.swift	91.48% <100%> (ø)	⬆️
Sources/XMLCoder/Box/IntBox.swift	85.71% <100%> (ø)	⬆️
Sources/XMLCoder/Box/DecimalBox.swift	85.71% <100%> (ø)	⬆️
Sources/XMLCoder/Box/StringBox.swift	100% <100%> (ø)	⬆️
... and 12 more

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