[v3] Extract Concepts from v2 exercise: pascals-triangle

Extract the list of concepts required to solve the v2 purescript exercise: `pascals-triangle`. For each concept, describe why it is required to solve the exercise. Keep in mind that an exercise can have multiple approaches (you can find community solutions here: `https://exercism.io/tracks/purescript/exercises/pascals-triangle/solutions`). The goal of this task is to help determine the total list of concepts that make up `pascals-triangle`.

The list of concepts should be stored in the following document: `languages/purescript/reference/exercise-concepts/pascals-triangle.md`. [Use this link to create a PR to add this document][pr-link].

[pr-link]: ../new/master?filename=languages/purescript/reference/exercise-concepts/pascals-triangle.md