feat(challenges): add consecutive-numbers

challenges/consecutive-numbers/README.md

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+# consecutive-numbers
+
+Created by [@Divlo](https://github.com/Divlo) on 28 June 2021.
+
+## Instructions
+
+Write a function which takes a list of integers, and which returns the (possibly empty) list of pairs of successive consecutive integers that there may be in the list.
+
+### Input
+
+```txt
+7
+1
+2
+5
+3
+4
+```
+
+### Output
+
+```txt
+1, 2
+3, 4
+```
+
+## Examples
+
+See the `test` folder for examples of input/output.