feat: allow variable declarations without explicit initializers

In the compiler, when parsing a variable declaration, the `=` token is now optional. If present, the initializer expression is compiled as before. If absent, the variable is implicitly initialized to `null` via a call to `null(compiler, false)`. This change removes the previous `TODO` comment and the unconditional `consume` of `TOKEN_EQ`.

Two new test files verify the behavior for both global and local variables without initializers, asserting that reading such a variable yields `null`.

test/variable/global_without_initializer.wren

@@ -0,0 +1,2 @@
+var a
+IO.write(a) // expect: null

test/variable/local_without_initializer.wren

@@ -0,0 +1,4 @@
+{
+  var a
+  IO.write(a) // expect: null
+}