Added elements for listing child pages and section navigation

[mak001]

• like the ChildPagesBlock and SectionNavigationBlock

Reorganized tests to follow code directory structure

[codecov]

Codecov Report

Merging #10 into master will decrease coverage by 1%.
The diff coverage is 30%.

@@             Coverage Diff              @@
##             master      #10      +/-   ##
============================================
- Coverage     49.45%   48.45%   -1.01%     
- Complexity       57       62       +5     
============================================
  Files             9       11       +2     
  Lines           184      194      +10     
============================================
+ Hits             91       94       +3     
- Misses           93      100       +7

Impacted Files	Coverage Δ	Complexity Δ
src/Elements/ElementChildPages.php	0% <0%> (ø)	1 <1> (?)
src/Elements/ElementSectionNavigation.php	37.5% <37.5%> (ø)	4 <4> (?)

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