Collapsible DOM updates

[yairEO]

• Added namespace constant for dynamic selectors names
• Refactored selectors and changed to BEM naming, for improved isolation
• Tests now using the (exported) namespace
• TBD: use dynamic prefix imported from a centralized place

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All examples have been tested manually.

[codecov[bot]]

Codecov Report

Merging #95 (50c24f5) into 2.x (a806747) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              2.x      #95   +/-   ##
=======================================
  Coverage   96.11%   96.12%           
=======================================
  Files          62       62           
  Lines         850      852    +2     
  Branches       29       29           
=======================================
+ Hits          817      819    +2     
  Misses         32       32           
  Partials        1        1           

Impacted Files	Coverage Δ
src/components/Collapsible/Collapsible.jsx	95.83% <100.00%> (+0.37%)	:arrow_up:

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