printing with styled/non-unit-textwidth-character contents

[thchr]

Previously, BlockArrays with elements that used styling would throw off the printing, because the textwidth computation didn't account for the fact that some contents might have a different length relative to how much space they take up when printed.

This fixes that by using the ANSIIterator from Base, ala JuliaLang/julia#47430. Unfortunately, ANSIIterator isn't exported at the moment - but the alternative is to depend on StringManipulations.jl or copy over the Base implementation.

I added a test that shows the problem. Previously, it displayed as:

With this, it displays as:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.55%. Comparing base (4d386cf) to head (4ea3b1d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #443   +/-   ##
=======================================
  Coverage   93.55%   93.55%           
=======================================
  Files          18       18           
  Lines        1644     1644           
=======================================
  Hits         1538     1538           
  Misses        106      106           

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