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/return/after_else.wren

@@ -1,3 +1,3 @@
-IO.print(new Fn {
+IO.print(Fn.new {
   if (false) "no" else return "ok"
 }.call()) // expect: ok

test/language/return/after_if.wren

@@ -1,3 +1,3 @@
-IO.print(new Fn {
+IO.print(Fn.new {
   if (true) return "ok"
 }.call()) // expect: ok

test/language/return/after_while.wren

@@ -1,3 +1,3 @@
-IO.print(new Fn {
+IO.print(Fn.new {
   while (true) return "ok"
 }.call()) // expect: ok

test/language/return/in_function.wren

@@ -1,4 +1,4 @@
-var f = new Fn {
+var f = Fn.new {
   return "ok"
   IO.print("bad")
 }

test/language/return/in_method.wren

@@ -5,4 +5,4 @@ class Foo {
   }
 }
 
-IO.print((new Foo).method) // expect: ok
+IO.print(Foo.new().method) // expect: ok

test/language/return/return_null_if_newline.wren

@@ -1,4 +1,4 @@
-var f = new Fn {
+var f = Fn.new {
   return
   IO.print("bad")
 }