refactor: improve groupBy

[ST-DDT]

Adds a value mapping function to groupBy.

Intended for #2530

[codecov]

Codecov Report

Merging #2532 (cab1af5) into next (9b00fe9) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             next    #2532   +/-   ##
=======================================
  Coverage   99.56%   99.57%           
=======================================
  Files        2805     2805           
  Lines      250097   250126   +29     
  Branches     1099     1103    +4     
=======================================
+ Hits       249018   249054   +36     
+ Misses       1051     1044    -7     
  Partials       28       28           

Files	Coverage Δ
src/internal/group-by.ts	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[xDivisionByZerox]

I feel like this missed the point of "grouping" by a key. This function would then break the single responsibility principle.

Note that the following polifills (which this function was inspired by) do not include a value mapper as well:

• Object.groupBy
• Map.groupBy
• Array.prototype.groupToMap

[ST-DDT]

In the Java world they noticed quite early (Java 8), that there is a need to give the user the ability to also process/transform the value.
Common usecases include wrapper shedding, which is the reason why I added the parameter in the first place.

groupBy(
  [
    [1, "someValue"],
    [2, "blub"],
    [1, "anotherone"]
  ], 
  (e) => e[0],
  (e) => e[1]
);

=>

{
  1: ["someValue", "anotherone"],
  2: ["blub"]
}

I can explain this in more detail in the next team-meeting if you would like to.

[xDivisionByZerox]

Please add the behaviour to the indended place as #2530 is already merged. I'm fine with keeping the additional utility.

[xDivisionByZerox]

"Off-topic" question: Should we have tests for our internal function?