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/static.wren

@@ -6,7 +6,7 @@ class Foo {
   static bar(arg) { "on metaclass " + arg }
 }
 
-IO.print((new Foo).bar)        // expect: on instance
+IO.print(Foo.new().bar)        // expect: on instance
 IO.print(Foo.bar)              // expect: on metaclass
-IO.print((new Foo).bar("arg")) // expect: on instance arg
+IO.print(Foo.new().bar("arg")) // expect: on instance arg
 IO.print(Foo.bar("arg"))       // expect: on metaclass arg