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/assign_global.wren

@@ -0,0 +1,8 @@
+var a = "before"
+io.write(a) // expect: before
+
+a = "after"
+io.write(a) // expect: after
+
+io.write(a = "arg") // expect: arg
+io.write(a) // expect: arg