Fix: Reject mixed tabs and spaces as indent

[localheinz]

This PR

• asserts that mixed space and tabs are rejected as indent values
• adjusts a regular expression

Follows ergebnis/json-normalizer#69.

[codecov]

Codecov Report

Merging #37 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #37   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity       20     20           
=======================================
  Files             1      1           
  Lines            53     53           
=======================================
  Hits             53     53

Impacted Files	Coverage Δ	Complexity Δ
src/Printer.php	100% <100%> (ø)	20 <0> (ø)	⬇️

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