CLI: improve diagnostic message format

- render the whole line where the error/warning points to, if any, and another
  line with '^', like other compilers do
- lowercase diagnostic severity
- decapitalize the message if it doesn't start with a proper name

compiler/testData/repl/objects/localObject.repl

@@ -2,4 +2,6 @@
 ...   object Bar {
 ...   }
 ... }
-ERROR: /line4.kts: (2, 1) Named object 'Bar' is a singleton and cannot be local. Try to use anonymous object instead
+/line4.kts:2:1: error: named object 'Bar' is a singleton and cannot be local. Try to use anonymous object instead
+object Bar {
+^