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/implicit_receiver/inherited_methods.wren

@@ -20,4 +20,4 @@ class Bar is Foo {
   }
 }
 
-(new Bar).test
+Bar.new().test

test/language/implicit_receiver/instance_methods.wren

@@ -18,6 +18,6 @@ class Foo {
   }
 }
 
-(new Foo).test
+Foo.new().test
 
 // TODO: Need to decide how these interact with globals.

test/language/implicit_receiver/locals_shadow_getter.wren

@@ -14,4 +14,4 @@ class Foo {
   }
 }
 
-(new Foo).test
+Foo.new().test

test/language/implicit_receiver/locals_shadow_setter.wren

@@ -17,4 +17,4 @@ class Foo {
   }
 }
 
-(new Foo).test
+Foo.new().test

test/language/implicit_receiver/nested_class.wren

@@ -36,7 +36,7 @@ class Outer {
       }
     }
 
-    (new Inner).test
+    Inner.new().test
 
     getter            // expect: outer getter
     setter = "value"  // expect: outer setter
@@ -44,4 +44,4 @@ class Outer {
   }
 }
 
-(new Outer).test
+Outer.new().test