Improve the way mix is invoked in concurrent VSCode environment

[frantic]

Looks like there was a change in how VSCode works.

In the past this extension used to call document.save() before invoking mix format. The format function simply operated on saved file.

Now VSCode invokes the extension before it saves the file, and document.save() waits for this extension to finish, but the extension is waiting for document.save(). VSCode shows infinite spinner dialog.

This PR fixes the behavior. I switched exec with spawn and use stdio to send the unformatted and receive the formatted file. I've also improved the error message so it should be easier to figure out what's not working.

Because I had to make some substantial changes, I also backported #6 by @maennchen (which fixes #5). #4 should be fixed too because spawn accepts arguments as array of strings and does the escaping for us.

I've tried my best to do some reasonable testing. mixFormatArgs and formatterCwd options are still working and I documented the new mixCommandPath.

[bitboxer]

@sarat-ravi is this still maintained? Would love to see this merged and would offer any help you need for this.

[psylone]

Now VSCode invokes the extension before it saves the file, and document.save() waits for this extension to finish, but the extension is waiting for document.save(). VSCode shows infinite spinner dialog.

I wonder, whether this bug occurs always or in case when Format On Save option enabled?