Refactor CI workflow

[jgarber623]

Description

This PR renames and refactors the project's GitHub Actions CI workflow. The updated workflow will now run on:

• pushes to main,
• pull requests against main, and
• by manual invocation via GitHub's UI

Notable changes to the workflow include:

• use CargoSense/setup-elixir-project reusable workflow (in Ubuntu job, at least)
• adds Ubuntu and macOS to the test matrix
• refactors existing Ubuntu test job
• adds a new macOS test job
• specifies oldest and newest Elixir and Erlang combo in the matrix
• use latest actions/checkout
• sets job name to use matrix values for clarity

The macOS job takes a necessarily different tact for installing Erlang and Elixir owing to the underlying erlef/setup-beam workflow not (yet) supporting macOS (see https://github.com/erlef/setup-beam/issues/54).

[jgarber623]

Error on the macOS job:

Error: @erlef/setup-beam only supports Ubuntu and Windows at this time

😭