Set the working directory for commands with the cwd key

[w0rp]

Related to #2256.

Vim patch 8.0.0902 adds an cwd key for setting the working directory for jobs, and NeoVim offers a similar option. We could set the working directory this way to make command lines shorter on Windows, provided we use a version check and fall back on editing the command in other cases.

[w0rp]

Having thought about this more, and considering functionality ALE supports for running commands with wrapper scripts and on other machines, such as inside a Docker image or in a VM, using the actual cwd keys for the job functions will provide less functionality than we already have, and we'll also need to provide fallbacks for older Vim versions we want to continue to support anyway.

However, splitting the working directory from the commands users specify and then putting the cd command at the front of the command we actually run has a lot of value, because it will allow us to swap out the working directory when running checks for entire projects. See #3599. I'll do that instead.

[w0rp]

I've implemented this now with my new idea. Keeping cd ... && in the command will make it easy to support changing directory inside of another machine in future with filename mapping. The next step is to implement a command for running ALE linters over an entire project.