ColourUtil tests and small refactors

[AydinHassan]

• used is_numeric instead of is_int so we don't have to mix up types, we can just let php cast for us now, this allows us to keep parameter types and return types.

• fixed an issue with generating the colour set code when the bg colour hasn't been set yet

• Added tests for ColourUtil

• Refactored validateColour to reduce nesting and return early

[codecov-io]

Codecov Report

Merging #107 into master will increase coverage by 0.43%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #107      +/-   ##
============================================
+ Coverage     96.32%   96.76%   +0.43%     
+ Complexity      307      306       -1     
============================================
  Files            23       23              
  Lines           926      928       +2     
============================================
+ Hits            892      898       +6     
+ Misses           34       30       -4

Impacted Files	Coverage Δ	Complexity Δ
src/Util/ColourUtil.php	100% <100%> (+23.52%)	8 <6> (-1)	⬇️
src/CliMenuBuilder.php	98.69% <100%> (ø)	47 <2> (ø)	⬇️
src/MenuStyle.php	94.73% <100%> (+0.04%)	39 <2> (ø)	⬇️

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