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/newlines.wren

@@ -3,7 +3,7 @@ class Foo {
   [a, b] { "subscript " + a + " " + b }
 }
 
-var foo = new Foo
+var foo = Foo.new()
 
 // Allow newlines after commas and before ")".
 IO.print(foo.method("a",