Simple string interpolation.

This allows "%(...)" inside a string literal to interpolate the
stringified result of an expression.

It doesn't support custom interpolators or format strings, but we can
consider extending that later.

test/limit/interpolation_nesting.wren

@@ -0,0 +1 @@
+System.print("0 %("1 %("2 %("3 %("4 %("5 %("6 %("7 %(8)")")")")")")")") // expect: 0 1 2 3 4 5 6 7 8

test/limit/too_much_interpolation_nesting.wren

@@ -0,0 +1 @@
+System.print("0 %("1 %("2 %("3 %("4 %("5 %("6 %("7 %("8 %(9)")")")")")")")")") // expect error