Add IsSubstringOf assertion

[mzampetakis]

No description provided.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[codecov]

Codecov Report

Merging #45 (9d3a31c) into master (47d1f22) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #45      +/-   ##
==========================================
+ Coverage   97.53%   97.56%   +0.02%     
==========================================
  Files          12       12              
  Lines         650      657       +7     
==========================================
+ Hits          634      641       +7     
  Misses         16       16              

Impacted Files	Coverage Δ
assert/error_messages.go	100.00% <100.00%> (ø)
assert/string.go	100.00% <100.00%> (ø)

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[ppapapetrou76]

LGTM