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/core/object/same.wren

@@ -26,15 +26,15 @@ IO.print(Object.same(Bool, Bool)) // expect: true
 // Other types compare by identity.
 class Foo {}
 
-var foo = new Foo
+var foo = Foo.new()
 IO.print(Object.same(foo, foo)) // expect: true
-IO.print(Object.same(foo, new Foo)) // expect: false
+IO.print(Object.same(foo, Foo.new())) // expect: false
 
 // Ignores == operators.
 class Bar {
   ==(other) { true }
 }
 
-var bar = new Bar
+var bar = Bar.new()
 IO.print(Object.same(bar, bar)) // expect: true
-IO.print(Object.same(bar, new Bar)) // expect: false
+IO.print(Object.same(bar, Bar.new())) // expect: false