housekeeping: update the git ignore file.

[glennawatson]

Update .gitignore file to match the ReactiveUI one.

[codecov]

Codecov Report

Merging #375 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #375   +/-   ##
=======================================
  Coverage   71.33%   71.33%           
=======================================
  Files          66       66           
  Lines        3663     3663           
  Branches      333      333           
=======================================
  Hits         2613     2613           
  Misses       1004     1004           
  Partials       46       46

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