Add code exists check

[mikeymike]

Adds a new check to see if the solution has any actual statements.

[codecov-io]

Codecov Report

Merging #197 (c37f8a6) into master (dc62895) will increase coverage by 0.01%.
The diff coverage is 95.23%.

@@             Coverage Diff              @@
##             master     #197      +/-   ##
============================================
+ Coverage     91.75%   91.77%   +0.01%     
- Complexity      677      692      +15     
============================================
  Files           100      101       +1     
  Lines          2087     2116      +29     
============================================
+ Hits           1915     1942      +27     
- Misses          172      174       +2     

Impacted Files	Coverage Δ	Complexity Δ
src/ExerciseRunner/CgiRunner.php	90.74% <ø> (ø)	25.00 <0.00> (ø)
src/ExerciseRunner/CliRunner.php	100.00% <ø> (ø)	20.00 <0.00> (ø)
src/Check/CodeExistsCheck.php	95.23% <95.23%> (ø)	13.00 <13.00> (?)
src/functions.php	57.14% <0.00%> (-9.53%)	0.00% <0.00%> (ø%)
src/Utils/StringUtils.php	100.00% <0.00%> (ø)	9.00% <0.00%> (+2.00%)

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