Feature/node container css classes

[jacksonjunelee]

I have been having trouble adding custom css and custom classes to the value container of the node. This PR allows people to add custom classes to each node through the TreeModel settings

[codecov]

Codecov Report

Merging #284 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
+ Coverage   92.13%   92.16%   +0.03%     
==========================================
  Files          25       25              
  Lines        1030     1034       +4     
  Branches      146      147       +1     
==========================================
+ Hits          949      953       +4     
  Misses         53       53              
  Partials       28       28

Impacted Files	Coverage Δ
src/tree.types.ts	100% <ø> (ø)	⬆️
src/tree.ts	92.88% <100%> (+0.06%)	⬆️
src/tree-internal.component.ts	83.06% <100%> (+0.18%)	⬆️

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