feat(findspairs): new exam exercise

subjects/findpairs/README.md

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+## Find Pairs with Sum
+
+### Instructions
+
+Write a program that finds all pairs of elements in an integer array that sum up to a given target value. The program should output a list of pairs, each representing the indices of the elements that form the pair.
+
+In this exercise you must take in consideration the following:
+
+- Ensure its possible to have positive or negative integers in the array
+- Ensure each element is used only once in a pair
+- Allow for multiple pairs to sum up to the target value
+- Return the message "No pairs found." when no pair is present
+- Return the message "Invalid target sum." if the target is invalid
+- Return the message "Invalid number." if the number in the array is invalid
+
+Let's consider the input `arr = [1, 2, 3, 4, 5]` and the target sum `targetSum = 6`. When we run the program, the `findPairs()` function will search for pairs in the array that sum up to `targetSum`.
+
+The expected output for this input will be:
+
+```console
+$ go run . "[1, 2, 3, 4, 5]" "6"
+Pairs with sum 6: [[0 4] [1 3]]
+$ go run . "[-1, 2, -3, 4, -5]" "1"
+Pairs with sum 1: [[0 1] [2 3]]
+$ go run . "[1, 2, 3, 4, 5]" "10"
+No pairs found.
+$ go run . "[1, 2, 3, 4, 20, -4, 5]" "2 5"
+Invalid target sum.
+$ go run . "[1, 2, 3, 4, 20, p, 5]" "5"
+Invalid number: p
+$
+```