## Find Pairs

### 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 it's possible to have positive or negative integers in the array.
- Ensure each element is used only once in a pair, although the element can be repeated in different pairs.
- Allow for multiple pairs to sum up to the target value.
- The output messages should follow the one given in the examples bellow.
- 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: <number>"** if the number in the array is invalid.
- For any input format that deviates from the specified format `"[1, 2, 3, 4, 5]" "6"`, the program will return an "Invalid input." error message.

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`.

Here is some example of outputs:

```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, -5]" "-5"
Pairs with sum -5: [[0 3] [1 2]]
$ 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
$ go run . "[1, 2, 3, 4" "5"
Invalid input.
$ go run . "1, 2, 3, 4" "5"
Invalid input.
$
```