🐛 [RUMF-469] fix angular compatibility

[BenoitZugmeyer]

Angular uses Zone.js to provide a context persisting accross async tasks. Zone.js replaces the global MutationObserver constructor with a patched version to support the context propagation. There is an ongoing issue[1, 2] with this setup when using a MutationObserver within a Angular component: on some occasions, the callback is being called in an infinite loop, causing the page to freeze (even if the callback is completely empty).

To work around this issue, we are using the Zone __symbol__ API to get the original, unpatched MutationObserver constructor.

[codecov-io]

Codecov Report

Merging #376 into master will decrease coverage by 0.16%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #376      +/-   ##
==========================================
- Coverage   86.94%   86.78%   -0.17%     
==========================================
  Files          30       30              
  Lines        1693     1702       +9     
  Branches      343      345       +2     
==========================================
+ Hits         1472     1477       +5     
- Misses        221      225       +4     

Impacted Files	Coverage Δ
packages/rum/src/domMutationCollection.ts	89.47% <80.00%> (-10.53%)	⬇️
packages/rum/src/rum.ts	85.10% <0.00%> (-2.13%)	⬇️

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