feat: update Problem to account for related information and code

This PR makes changes to the existing `xsbti.Problem` to account for an
optional diagnostic code that the compiler can return for a given
diagnostic and also related information.

Given a piece of code like:

```scala
try {}
```

You'll receive the following:

```
-- [E002] Syntax Warning: /Users/ckipp/Documents/scala-workspace/dotty-error-index/examples/002_EmptyCatchAndFinallyBlockID.scala:3:2
3 |  try {}
  |  ^^^^^^
  |  A try without catch or finally is equivalent to putting
  |  its body in a block; no exceptions are handled.
```

The `E002` here is the actual code. Right now there would be no
description.

Some diagnostics have multiple positions that they need to represent.
You can see an example of this
[here](lampepfl/dotty#14002) in Dotty with the
use of inlining. Instead of needing to rely on including all of that
information in the diagnostic message it can now be extracted out into
a `DiagnosticRelatedInformation`.

These changes reference the conversation in #6868

internal/util-interface/src/main/java/xsbti/DiagnosticCode.java

@@ -0,0 +1,23 @@
+/*
+ * sbt
+ * Copyright 2011 - 2018, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+
+package xsbti;
+
+import java.util.Optional;
+
+/**
+ * A DiagnosticCode is a unique identifier that the compiler can associate with a diagnostic. This
+ * is useful for tools to be able to quickly identify what diagnostic is being reported without
+ * having to rely on parsing the actual diagnostic message, which might not be stable.
+ */
+public interface DiagnosticCode {
+  /** The unique code. This is typically in the format of E000 */
+  String code();
+
+  /** Possible explanation to explain the meaning of the code */
+  Optional<String> explanation();
+}