Fix handling of line break characters in a string in pretty printing

[lgoettgens]

Resolves oscar-system/Oscar.jl#3515.

The field io.printed should keep track of the width already printed in the current line (indentation is not tracked here).
In two places, a newline char was printed, but the field was not reset to 0.

There is unfortunately no way to test this in a doctest, as all of this linebreaking business is skipped for doctests.

In local testing, this both resolves the MWE in oscar-system/Oscar.jl#3515 (comment), and the printing issues with MonomialBasis (at least those that I could observe locally).

cc @benlorenz @lkastner

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 87.00%. Comparing base (7e756b8) to head (74fca7e).

Files	Patch %	Lines
src/PrettyPrinting.jl	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1636      +/-   ##
==========================================
+ Coverage   86.94%   87.00%   +0.05%     
==========================================
  Files         114      114              
  Lines       29700    29707       +7     
==========================================
+ Hits        25823    25846      +23     
+ Misses       3877     3861      -16     

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[benlorenz]

Thanks! Looks correct now (also when passing through our fake repl pipes).