feat: add subscript operator for array indexing and string character access

Implement the `[]` subscript operator as an infix parser at `PREC_CALL` precedence, compiling to a method call on the receiver. Add a `string_subscript` primitive that handles integer indices (including negative for reverse indexing), returns `null` for out-of-bounds or non-integer arguments, and returns a one-character string. Register the `[ ]` method on the string class and include tests for basic indexing, negative indices, out-of-bounds, wrong types, and non-integer indices.

test/string_subscript.wren

@@ -0,0 +1,24 @@
+// Returns characters (as strings).
+io.write("abcd"[0]) // expect: a
+io.write("abcd"[1]) // expect: b
+io.write("abcd"[2]) // expect: c
+io.write("abcd"[3]) // expect: d
+
+// Allows indexing backwards from the end.
+io.write("abcd"[-4]) // expect: a
+io.write("abcd"[-3]) // expect: b
+io.write("abcd"[-2]) // expect: c
+io.write("abcd"[-1]) // expect: d
+
+// Handle out of bounds.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write("abcd"[4])  // expect: null
+io.write("abcd"[-5]) // expect: null
+
+// Handle wrong argument type.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write("abcd"[true]) // expect: null
+
+// Handle non-integer index.
+// TODO(bob): Should halt the fiber or raise an error somehow.
+io.write("abcd"[1.5]) // expect: null