feat: add user-defined subscript operators with bracket parameter lists

Refactor parameter list parsing to support bracket-delimited parameters for
subscript operators. Extract finishParameterList from parameterList to handle
the common closing logic, and add TOKEN_RIGHT_BRACKET case for error messages.
Add comprehensive test suite for subscript getters and setters with 1-3
parameters in test/method/subscript_operators.wren.

test/list/subscript_setter.wren

@@ -21,5 +21,3 @@
   list[-3] = 7
   IO.print(list) // expect: [7, 6, 5]
 }
-
-// TODO: Not in this dir, but need tests for subscript setter grammar.

test/method/subscript_operators.wren

@@ -0,0 +1,16 @@
+class Foo {
+  [a] { "1-subscript " + a }
+  [a, b] { "2-subscript " + a + " " + b }
+  [a, b, c] { "3-subscript " + a + " " + b + " " + c }
+  [a]=(value) { "1-subscript setter " + a + " = " + value }
+  [a, b]=(value) { "2-subscript setter " + a + " " + b + " = " + value }
+  [a, b, c]=(value) { "3-subscript setter " + a + " " + b + " " + c + " = " + value }
+}
+
+var foo = new Foo
+IO.print(foo["a"]) // expect: 1-subscript a
+IO.print(foo["a", "b"]) // expect: 2-subscript a b
+IO.print(foo["a", "b", "c"]) // expect: 3-subscript a b c
+IO.print(foo["a"] = "value") // expect: 1-subscript setter a = value
+IO.print(foo["a", "b"] = "value") // expect: 2-subscript setter a b = value
+IO.print(foo["a", "b", "c"] = "value") // expect: 3-subscript setter a b c = value