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

test/language/nonlocal/mutual_recursion.wren

@@ -1,9 +1,9 @@
 class Foo {
-  static bar { new Bar }
+  static bar { Bar.new() }
 }
 
 class Bar {
-  static foo { new Foo }
+  static foo { Foo.new() }
 }
 
 IO.print(Foo.bar) // expect: instance of Bar

test/language/nonlocal/undefined.wren

@@ -1,7 +1,6 @@
-var fn = new Fn {
+var fn = Fn.new {
   IO.print(Foo)
   IO.print(Bar)
 }
 
-// expect error line 7
-// expect error line 7
+// expect error line 6

test/language/nonlocal/use_in_function.wren

@@ -1,5 +1,5 @@
 var Global = "global"
 
-new Fn {
+Fn.new {
   IO.print(Global) // expect: global
 }.call()

test/language/nonlocal/use_in_function_before_definition.wren

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

test/language/nonlocal/use_in_method.wren

@@ -10,5 +10,5 @@ class Foo {
   }
 }
 
-(new Foo).method // expect: global
+Foo.new().method // expect: global
 Foo.classMethod // expect: global

test/language/nonlocal/use_in_method_before_definition.wren

@@ -10,5 +10,5 @@ class Foo {
 
 var Global = "global"
 
-(new Foo).method // expect: global
+Foo.new().method // expect: global
 Foo.classMethod // expect: global