Support multiline string literals

[kishikawakatsumi]

No description provided.

[codecov]

Codecov Report

Merging #58 into master will increase coverage by 0.35%.
The diff coverage is 95.62%.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   78.14%   78.49%   +0.35%     
==========================================
  Files          17       17              
  Lines        4941     5041     +100     
  Branches      538      542       +4     
==========================================
+ Hits         3861     3957      +96     
- Misses       1078     1082       +4     
  Partials        2        2

Impacted Files	Coverage Δ
Tests/SwiftPowerAssertTests/AssertTests.swift	99.51% <100%> (+0.01%)	⬆️
Sources/SwiftPowerAssertCore/Transformer.swift	96.47% <92%> (-0.23%)	⬇️
Sources/SwiftPowerAssertCore/Formatter.swift	92.91% <96%> (+0.48%)	⬆️

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