feat: add unary operators and variable assignment with precedence handling

Implement unary negation for numbers and logical not for booleans, introduce PREC_ASSIGNMENT and PREC_UNARY precedence levels, refactor parse functions to accept allowAssignment parameter, add assignment tests for global/local/chained/error cases, and update if-syntax to support statement branches and assignment in conditions.

test/number_minus.wren

@@ -1,5 +1,10 @@
+// Infix.
 io.write(5 - 3)      // expect: 2
 io.write(3.1 - 0.24) // expect: 2.86
 io.write(3 - 2 - 1)  // expect: 0
 
+// Unary negation.
+var a = 3
+io.write(-a) // expect: -3
+
 // TODO(bob): Unsupported RHS types.