feat: replace new keyword with this constructor syntax and ClassName.new() calls across core library and docs

Replace all uses of the `new` reserved word with explicit `this new()` constructor definitions and `ClassName.new()` instantiation calls in builtin/core.wren. Update all documentation files (classes.markdown, fiber.markdown, fn.markdown, sequence.markdown, error-handling.markdown, expressions.markdown, fibers.markdown, functions.markdown) to reflect the new constructor syntax, removing `new` keyword examples and replacing them with `ClassName.new()` patterns and `this new()` definitions.

test/language/method/subscript_operators.wren

@@ -7,7 +7,7 @@ class Foo {
   [a, b, c]=(value) { "3-subscript setter " + a + " " + b + " " + c + " = " + value }
 }
 
-var foo = new Foo
+var foo = Foo.new()
 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