Fix: Sniff only pure indents, no mixed spaces and tabs

[localheinz]

This PR

• asserts that sniffed indents are pure (that is, either space or tab, but not mixed)
• adjusts a regular expression

Related to #69.

[codecov]

Codecov Report

Merging #71 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #71   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity      108    108           
=======================================
  Files            13     13           
  Lines           314    314           
=======================================
  Hits            314    314

Impacted Files	Coverage Δ	Complexity Δ
src/Format/Sniffer.php	100% <100%> (ø)	12 <0> (ø)	⬇️

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