Add task to run test at cursor

[manzanit0]

Adds a task to the extension to run the current test under the cursor.

I wasn't too confident on adding a default keybinding because I'm not familiar with any policies around keybindings in ElixirLS project. If you think it's worth, it can be done too. I currently have it set up to ctrl + '.

Below is a screenshot of the task selector + execution.

[rossvz]

This is great! Any chance to get options to run the whole file/test suite too? I know theres another VS Code extension that does this for elixir, but having this in one place feels nice

[robsonpeixoto]

Great! The vscode already have testing.runAtCursor, and other commands. What do yout think about implement the Testing API?

• https://github.com/microsoft/vscode-extension-samples/tree/main/test-provider-sample
• https://code.visualstudio.com/api/extension-guides/testing

[lukaszsamson]

Superceded by https://github.com/elixir-lsp/vscode-elixir-ls/pull/290