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/inheritance/is.wren

@@ -1,9 +1,9 @@
 class A {}
 class B is A {}
 class C is B {}
-var a = new A
-var b = new B
-var c = new C
+var a = A.new()
+var b = B.new()
+var c = C.new()
 
 IO.print(a is A) // expect: true
 IO.print(a is B) // expect: false