types(collection): reduce* method signatures

[Renegade334]

Please describe the changes this PR makes and why it should be merged:

Small changes relating to the typed method definitions for reduce() and reduceRight():

• Fix the "without-initial-value" signature of reduceRight().
  • When initialValue was not provided, this method would previously accept a callback with any arbitrary type for previousValue, or infer the type as unknown without explicit type annotation:

    declare const coll: Collection<string, number>;
    coll.reduceRight((a, x) => a + x); // error: 'a' is of type 'unknown'
    coll.reduceRight((a: string, x) => a.repeat(x)); // compiles OK, but will raise a TypeError at runtime

  • With this change, if an initial value for the accumulator is not provided, the callback's previousValue type is always the collection value type, and will raise an error otherwise.
• Separate out "accumulator type = collection value type" and "accumulator type = arbitrary type" overload signatures, à la TypeScript's library definitions for Array.prototype.reduce.
  • This changes the type behaviour in one specific set of cases, which is where a value for initialValue is provided with a type that is assignable to, but not equal to, the collection value type:

    declare const coll: Collection<string, number>;
    declare const initial: 1 | 2 | 3;
    
    coll.reduce((a, x) => a + x, initial); // Type 'number' is not assignable to type '1 | 2 | 3'

  • Previously, InitialValue would be inferred as 1 | 2 | 3, leading to compilation errors.
  • With this change, if initialValue is assignable to the collection value type, the method signature will preferentially use the collection value type for the callback, rather than narrowing to the type of initialValue. This matches the behaviour of Array.prototype.reduce in TypeScript.

Also adds one missing reduceRight() test (reduce empty collection with initial value).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.75%. Comparing base (bf6761a) to head (abe7cd0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10405      +/-   ##
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+ Coverage   50.73%   50.75%   +0.01%     
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  Files         228      228              
  Lines       20646    20652       +6     
  Branches     1253     1261       +8     
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+ Hits        10475    10481       +6     
  Misses      10127    10127              
  Partials       44       44              

Flag	Coverage Δ
collection	100.00% <100.00%> (ø)
rest	92.68% <ø> (ø)
ws	51.88% <ø> (ø)

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