Encode POSIX file path to URI using u3 (file:///)

Ref https://github.com/sbt/io/pull/96

Under RFC 8089, both u1 and u3 are legal, but many of the other platforms expect traditional u3.
This will increase the compatibility/usability of sbt server, for example to integrate with Vim.

main-command/src/main/scala/sbt/internal/server/Server.scala

@@ -174,7 +174,7 @@ private[sbt] object Server {
           auth match {
             case _ if auth(ServerAuthentication.Token) =>
               writeTokenfile()
-              PortFile(uri, Option(tokenfile.toString), Option(tokenfile.toURI.toString))
+              PortFile(uri, Option(tokenfile.toString), Option(IO.toURI(tokenfile).toString))
             case _ =>
               PortFile(uri, None, None)
           }