Add check for `increase_quantity` function on `boutique-inventory` exercise

[fmmatheus]

Issue: #199

Description

Add 2 checks for function increase_quantity on boutique-inventory concept exercise:

• Check if increase_quantity is using Enum.into
• Check if increase_quantity is not using Enum.map

This PR is based on a suggestion made by @jiegillet here: thread

Related PRs:

• Elixir: https://github.com/exercism/elixir/pull/973
• Website-copy: https://github.com/exercism/website-copy/pull/2104

[angelikatyborska]

I updated this PR to match the changes from https://github.com/exercism/elixir/pull/1101