fix: add .gitignore file to .elixir-tools directory

[lud-wj]

Fixes #118

[mhanberg]

You need to add .elixir-tools to your project's gitignore or your global gitignore.

[lud-wj]

Whenever I open an elixir project, and I have a lot, this directory will be created. Requiring users to update all their project's .gitignore files will lead to frustration, as some will inevitably forget it and start to add thousand files to their next commit.

A vscode extension should have no impact on the repository it is used in, in my opinion. This proposal aims to do just that : the directory ignores itself and everyone is happy.

[mhanberg]

That's why you can add it to your global gitignore.

But if this works, then might as well add it. I'll test it out tonight.