build: Add sbt-missinglink plugin

The plugin is quite immature so we cannot use it in 'check' command
but someone might run it from time to time to see any problems. Let's add
it to 'check' once it's ready to be used in production.

Author: jakubjanecek

project/plugins.sbt

@@ -6,3 +6,4 @@ addSbtPlugin("org.scalameta" % "sbt-mdoc" % "1.3.6")
 addSbtPlugin("com.geirsson" % "sbt-ci-release" % "1.4.31")
 addSbtPlugin("com.thoughtworks.sbt-api-mappings" % "sbt-api-mappings" % "3.0.0")
 addSbtPlugin("io.chrisdavenport" % "sbt-mima-version-check" % "0.1.2")
+addSbtPlugin("ch.epfl.scala" % "sbt-missinglink" % "0.1.0")