Implementing a VScode extension + a plugin mode for csharpier

[belav]

closes #283

This is the initial version of the vscode extension.
It supports

• installing csharpier globally if it is not
• formatting using csharpier < 0.12.0, which pipes a single file and is slower
• formatting using csharpier >= 0.12.0 with --pipe-multiple-files

This includes the following changes to csharpier

• adding --pipe-multiple-files to pipe multi files delimited by \u0003
• modifying csharpier to write to stderror when a file will not compile