Create a rum-core package

[BenoitZugmeyer]

Motivation

The rum-core package will be used by both rum and rum-recorder packages. It will mutualize RUM internals.

Changes

• Create a rum-core package
• Rename some 'global' to 'publicApi' to make things clearer

Testing

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I have gone over the contributing documentation.

[codecov-io]

Codecov Report

Merging #673 (fe3907f) into master (eb89c9b) will increase coverage by 0.13%.
The diff coverage is 94.25%.

@@            Coverage Diff             @@
##           master     #673      +/-   ##
==========================================
+ Coverage   77.72%   77.86%   +0.13%     
==========================================
  Files          60       60              
  Lines        3268     3266       -2     
  Branches      710      710              
==========================================
+ Hits         2540     2543       +3     
+ Misses        728      723       -5     

Impacted Files	Coverage Δ
packages/rum-core/src/boot/buildEnv.ts	100.00% <ø> (ø)
packages/rum-core/src/boot/rum.ts	50.00% <ø> (ø)
...ages/rum-core/src/browser/domMutationCollection.ts	89.47% <ø> (ø)
...ages/rum-core/src/browser/performanceCollection.ts	17.24% <ø> (ø)
packages/rum-core/src/domain/assembly.ts	100.00% <ø> (ø)
packages/rum-core/src/domain/internalContext.ts	100.00% <ø> (ø)
packages/rum-core/src/domain/lifeCycle.ts	100.00% <ø> (ø)
packages/rum-core/src/domain/parentContexts.ts	98.24% <ø> (ø)
packages/rum-core/src/domain/requestCollection.ts	87.87% <ø> (ø)
...ain/rumEventsCollection/action/actionCollection.ts	94.11% <ø> (ø)
... and 26 more

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[bcaudan]

LGTM