feat: implement expression evaluation and statement detection in REPL with Meta.compileExpression

Add executeInput() method to REPL that lexes input, determines if it's a statement or expression based on the first token, and evaluates accordingly using Meta.eval for statements or the new Meta.compileExpression for expressions. Extend Meta.compile_ foreign method with isExpression and printErrors parameters, update wrenCompile signature to support expression compilation mode, and add newline handling after equals in variable definitions.

test/language/variable/newline_after_equals.wren

@@ -0,0 +1,5 @@
+var foo =
+
+
+    123
+System.print(foo) // expect: 123