Add 'contains' matcher to VersionPattern

[fthomas]

Closes #1560.

[codecov]

Codecov Report

Merging #2365 (86e88cf) into master (0f3a69a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2365   +/-   ##
=======================================
  Coverage   80.26%   80.27%           
=======================================
  Files         139      139           
  Lines        2387     2388    +1     
  Branches       53       55    +2     
=======================================
+ Hits         1916     1917    +1     
  Misses        471      471           

Impacted Files	Coverage Δ
.../scalasteward/core/repoconfig/VersionPattern.scala	100.00% <100.00%> (ø)

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