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

test/assign_local.wren

@@ -0,0 +1,10 @@
+fn {
+  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
+}.call

test/assign_syntax.wren

@@ -0,0 +1,11 @@
+// Chained assignment.
+var a = "a"
+var b = "b"
+a = b = "chain"
+io.write(a) // expect: chain
+io.write(a) // expect: chain
+
+// Assignment on RHS of variable.
+var c = a = "var"
+io.write(a) // expect: var
+io.write(c) // expect: var

test/assign_to_grouping.wren

@@ -0,0 +1,2 @@
+var a = "a"
+(a) = "value" // expect error

test/assign_to_infix_operator.wren

@@ -0,0 +1,3 @@
+var a = "a"
+var b = "b"
+a + b = "value" // expect error

test/assign_to_is.wren

@@ -0,0 +1,3 @@
+var a = "a"
+var b = "b"
+b is a = "value" // expect error

test/assign_to_prefix_operator.wren

@@ -0,0 +1,2 @@
+var a = "a"
+!a = "value" // expect error

test/assign_undefined.wren

@@ -0,0 +1 @@
+unknown = "what" // expect error

test/bool_not.wren

@@ -0,0 +1,3 @@
+io.write(!true)    // expect: false
+io.write(!false)   // expect: true
+io.write(!!true)   // expect: true

test/grammar.wren

@@ -23,6 +23,9 @@ io.write(false == 2 <= 1) // expect: true
 // >= has higher precedence than ==.
 io.write(false == 1 >= 2) // expect: true
 
+// Unary - has lower precedence than ..
+io.write(-"abc".count) // expect: -3
+
 // TODO(bob): %, associativity.
 
 // Using () for grouping.

test/if_syntax.wren

@@ -6,11 +6,25 @@ if (false) io.write("bad")
 if (true) io.write("good") else io.write("bad") // expect: good
 if (false) io.write("bad") else io.write("good") // expect: good
 
+// Allow statements for then branch.
+if (true) { io.write("block") } // expect: block
+if (true) if (true) io.write("double") // expect: double
+
+// Allow statements for else branch.
+if (false) null else { io.write("block") } // expect: block
+if (false) null else if (true) io.write("double") // expect: double
+
 // Return the 'then' expression if the condition is true.
-io.write(if (true) "good") // expect: good
+var a = if (true) "good"
+io.write(a) // expect: good
 
 // Return null if the condition is false and there is no else.
-io.write(if (false) "bad") // expect: null
+var b = if (false) "bad"
+io.write(b) // expect: null
 
 // Return the 'else' expression if the condition is false.
-io.write(if (false) "bad" else "good") // expect: good
+var c = if (false) "bad" else "good"
+io.write(c) // expect: good
+
+// Assignment in if condition.
+if (a = true) io.write(a) // expect: true

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.