🐛 [RUMF-1305] forbid the usage of Date.now

[BenoitZugmeyer]

Motivation

Do not use Date.now because sometimes websites are wrongly "polyfilling" it. For example, we had some users using a very old version of datejs, which patched Date.now to return a Date instance instead of a timestamp. Those users are unlikely to fix this, so let's handle this case ourselves.

Changes

• Replace Date.now usages with a utility function
• Adjust the ESLint configuration to make sure we don't use Date.now in our source files

Testing

• Local
• Staging
• Unit
• End to end

---

I have gone over the contributing documentation.

[codecov-commenter]

Codecov Report

Merging #1600 (a275548) into main (7ab2389) will decrease coverage by 0.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##             main    #1600      +/-   ##
==========================================
- Coverage   90.84%   90.83%   -0.02%     
==========================================
  Files         123      123              
  Lines        4557     4559       +2     
  Branches     1021     1021              
==========================================
+ Hits         4140     4141       +1     
- Misses        417      418       +1     

Impacted Files	Coverage Δ
...ages/rum-core/src/browser/performanceCollection.ts	14.00% <0.00%> (ø)
...ages/core/src/domain/session/sessionCookieStore.ts	100.00% <100.00%> (ø)
packages/core/src/domain/session/sessionStore.ts	100.00% <100.00%> (ø)
packages/core/src/tools/timeUtils.ts	97.29% <100.00%> (-2.71%)	⬇️
.../rum-core/src/domain/tracing/getDocumentTraceId.ts	100.00% <100.00%> (ø)

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