Add support for cspell

[pbnj]

Name: cspell
URL: https://github.com/streetsidesoftware/cspell

From their repository:

A Spell Checker for Code!

cspell is a wildly popular, powerful, and configurable code spelling project with over 2 million downloads on Visual Studio Marketplace.

The cspell CLI is available via npm (e.g. npm install -g cspell) and outputs errorformat results out-of-the-box.

Example:

:cgetexpr system('npx cspell {*,**/*}.md')

Supporting cspell would allow for developers to share and re-use .cspell.json configurations.

[cyntheticfox]

I've been using cspell in VSCode and was wondering where it was in ALE... I might look at adding it if I can also get it added to nixpkgs...