Expose errorContextLength on struct XLSXFile

[MaxDesiatov]

New version of XMLCoder exposes errorContextLength, which could greatly help with diagnostics. There's now an additional argument with a default value of 0 exposed on XLSXFile initializer.

Resolves #32

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   98.18%   98.19%   +0.01%     
==========================================
  Files           5        5              
  Lines         220      222       +2     
==========================================
+ Hits          216      218       +2     
  Misses          4        4

Impacted Files	Coverage Δ
Sources/CoreXLSX/CoreXLSX.swift	97.64% <100%> (+0.05%)	⬆️

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