feat: add if/else expression parsing with jump bytecodes and null object support

Implement initial support for `if` expressions in the compiler, including new `TOKEN_IF` and `TOKEN_ELSE` tokens, `CODE_JUMP` and `CODE_JUMP_IF` bytecodes for conditional branching, and a `makeNull()` function in the VM to represent the value returned when an if-condition is false without an else clause. The `emit()` function now returns the index of the emitted bytecode to enable patching jump offsets. Also add a `nullClass` to the VM and a `OBJ_NULL` object type, along with a `dumpCode()` debugging helper and new test files for block syntax and if-expression behavior.

test/block_syntax.wren

@@ -0,0 +1,31 @@
+// Single line.
+{ io.write("ok") }.call // expect: ok
+
+// No trailing newline.
+{
+  io.write("ok") }.call // expect: ok
+
+// Trailing newline.
+{
+  io.write("ok") // expect: ok
+}.call
+
+// Multiple expressions.
+{
+  io.write("1") // expect: 1
+  io.write("2") // expect: 2
+}.call
+
+// Extra newlines.
+{
+
+
+  io.write("1") // expect: 1
+
+
+  io.write("2") // expect: 2
+
+
+}.call
+
+// TODO(bob): Arguments.

test/if_syntax.wren

@@ -0,0 +1,16 @@
+// Evaluate the 'then' expression if the condition is true.
+if (true) io.write("good") // expect: good
+if (false) io.write("bad")
+
+// Evaluate the 'else' expression if the condition is false.
+if (true) io.write("good") else io.write("bad") // expect: good
+if (false) io.write("bad") else io.write("good") // expect: good
+
+// Return the 'then' expression if the condition is true.
+io.write(if (true) "good") // expect: good
+
+// Return null if the condition is false and there is no else.
+io.write(if (false) "bad") // expect: null
+
+// Return the 'else' expression if the condition is false.
+io.write(if (false) "bad" else "good") // expect: good