Add the Dotty Language Server

This is an implementation of the Language Server Protocol for Dotty,
which allows us to provide rich IDE features in every editor supporting
this protocol, a Visual Studio Code extension demonstrating this is part
of the next commit. For more information on the LSP, see
https://github.com/Microsoft/language-server-protocol

Fully supported features:
- Typechecking as you type to show compiler errors/warnings
- Type information on hover
- Go to definition (in the current project)
- Find all references

Partially working features:
- Completion
- Renaming
- Go to definition in external projects

Unimplemented features:
- Documentation on hover
- Formatting code (requires integrating with scalafmt)
- Quick fixes (probably by integrating with scalafix)

Current limitations, to be fixed:
- Projects should be compiled with sbt before starting the IDE, this is
  automatically done for you if you run `sbt launchIDE`.
- Once the IDE is started, source files that are not opened in the IDE
  should not be modified in some other editor, the IDE won't pick up
  these changes.
- Not all compiler errors/warnings are displayed, just those occuring
  during typechecking.

Author: smarter

build.sbt

@@ -12,6 +12,7 @@ val `dotty-library-bootstrapped` = Build.`dotty-library-bootstrapped`
 val `dotty-sbt-bridge` = Build.`dotty-sbt-bridge`
 val `dotty-sbt-bridge-bootstrapped` = Build.`dotty-sbt-bridge-bootstrapped`
 val `dotty-sbt-scripted-tests` = Build.`dotty-sbt-scripted-tests`
+val `dotty-language-server` = Build.`dotty-language-server`
 val sjsSandbox = Build.sjsSandbox
 val `dotty-bench` = Build.`dotty-bench`
 val `scala-library` = Build.`scala-library`