fix(partitionByKey): unsubscribe from all streams when refcount reaches 0

[voliva]

I've found a couple of issues with partitionByKey:

• When the source completes, it completes the key$ and every instance observable. I think it should delegate this to the projection function so that when it completes, then the key is removed.
• After everyone unsubscribed from any of the streams it returns, it kept a subscription open to the inner observables.

I couldn't manage to change split/collect to solve these issues without breaking other split/collect tests, so I've reimplemented it as a standalone operator. I added a bunch of tests for added coverage, but the relevant ones are in these commit: https://github.com/re-rxjs/react-rxjs/commit/717b5dc4df853808e696147fa6cbfb7632971c20

• it keeps the existing keys alive when the source completes
• it unsubscribes from all streams when refcount reaches 0
• it emits a complete on the inner observable when the source completes
• it emits an error when the source errors

Edit: with this change we've decided to remove the old split/collect/collectValue methods

[codecov[bot]]

Codecov Report

Merging #202 (6e20cb2) into main (903c929) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #202   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        23    -4     
  Lines          448       422   -26     
  Branches        60        61    +1     
=========================================
- Hits           448       422   -26     

Impacted Files	Coverage Δ
packages/utils/src/internal-utils.ts	100.00% <100.00%> (ø)
packages/utils/src/partitionByKey.ts	100.00% <100.00%> (ø)

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