feat(challenges): add triangle-type

challenges/triangle-type/README.md

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+# triangle-type
+
+Created by [@Divlo](https://github.com/Divlo) on 30 June 2021.
+
+## Instructions
+
+Given the lengths of the 3 sides of a triangle, your function should return whether it is `equilateral`, `isosceles`, `scalene` or `impossible`.
+
+- A triangle is `equilateral` when its 3 sides are equal
+- A triangle is `isosceles` when 2 of its sides are equal
+- A triangle is `scalene` when none of its sides is equal to another side
+- A triangle is `impossible` when the sum of two of its sides is strictly less than the third side
+
+## Input
+
+- **Line 1 to 3:** The length of each side of the triangle
+
+## Examples
+
+See the `test` folder for examples of input/output.