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/limit/many_fields.wren

@@ -1,5 +1,5 @@
 class Foo {
-  new {
+  this new() {
     _field1 = 1
     _field2 = 2
     _field3 = 3
@@ -259,4 +259,4 @@ class Foo {
   }
 }
 
-var foo = new Foo // expect: 255
+var foo = Foo.new() // expect: 255