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/nonlocal/assignment.wren

@@ -12,7 +12,7 @@ class Foo {
 Foo.method
 IO.print(Nonlocal) // expect: method
 
-new Fn {
+Fn.new {
   Nonlocal = "fn"
 }.call()
 IO.print(Nonlocal) // expect: fn