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/language/interpolation/interpolation.wren

@@ -0,0 +1,15 @@
+// Full string.
+System.print("%(1 + 2)") // expect: 3
+
+// Multiple in one string.
+System.print("str%(1 + 2)(%(3 + 4)\%%(5 + 6)") // expect: str3(7%11
+
+// Nested.
+System.print("[%("{%("in" + "ner")}")]") // expect: [{inner}]
+
+// Ignore newlines in template.
+System.print("[%(
+
+"template"
+
+)]") // expect: [template]