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/class/lowercase_name_inside_body.wren

@@ -12,4 +12,4 @@ class foo {
 }
 
 foo.callFoo // expect: static foo method
-(new foo).callFoo // expect: instance foo method
+foo.new().callFoo // expect: instance foo method

test/language/class/name_inside_body.wren

@@ -11,4 +11,4 @@ class Foo {
 }
 
 Foo.sayName // expect: Foo!
-(new Foo).sayName // expect: Foo!
+Foo.new().sayName // expect: Foo!

test/language/class/new.wren

@@ -1,6 +1,6 @@
 class Foo {}
 
-var foo = new Foo
+var foo = Foo.new()
 IO.print(foo is Foo) // expect: true
 
 // TODO: Test precedence and grammar of what follows "new".