xsbti.Position: add startOffset and endOffset

A position now has a start, an end, and a point (the existing `offset`),
just like it does in the Scala compiler. This information is especially
useful for displaying squiggly lines in an IDE.

This commit and the next one are required for https://github.com/sbt/zinc/pull/571

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

@@ -18,4 +18,8 @@ public interface Position
 
 	Optional<String> sourcePath();
 	Optional<File> sourceFile();
+
+	// Default values to avoid breaking binary compatibility
+	default Optional<Integer> startOffset() { return Optional.empty(); }
+	default Optional<Integer> endOffset() { return Optional.empty(); }
 }