Add PriorityQueue function

[destel]

Add PriorityQueue function that allows injecting priority queues into pipelines.

The function takes a regular channel and returns a new one that emits items in priority order:

func PriorityQueue[A any](in <-chan Try[A], capacity int, less func(A, A) bool) <-chan Try[A]

Example usage:

// Process items from highest to lowest priority
itemsPrioritized := rill.PriorityQueue(items, 10, func(a, b Item) bool {
    return a.Priority > b.Priority
})

[codecov]

Codecov Report

Attention: Patch coverage is 74.26901% with 44 lines in your changes missing coverage. Please review.

Project coverage is 95.53%. Comparing base (b38495b) to head (f8e1135).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/heapbuffer/buffer.go	0.00%	24 Missing ⚠️
buffer.go	0.00%	11 Missing ⚠️
internal/core/buffer.go	81.25%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
- Coverage   99.11%   95.53%   -3.59%     
==========================================
  Files          17       21       +4     
  Lines        1015     1186     +171     
==========================================
+ Hits         1006     1133     +127     
- Misses          9       52      +43     
- Partials        0        1       +1     

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