Write a basic `leap` analysis

[angelikatyborska]

Write a basic check for the leap exercise that will check that the Erlang :calendar module is not used, as it is not allowed by the exercise's definition: https://github.com/exercism/elixir/blob/4e18d2385066e46de5ee5ea3e1ba9ce0caffdfa5/exercises/practice/leap/README.md#L16-L17

Introduction to writing an analyzer extension: https://github.com/exercism/elixir-analyzer/blob/main/docs/writing-an-analyzer.md

An example of an analyzer extension that detect usages of disallowed modules: https://github.com/exercism/elixir-analyzer/pull/50/files#diff-39ba60165e48ed55ff5dc41ce966786ae8f1c83abf1cc3e2430e0b8b5692f105