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/closure/close_over_later_variable.wren

@@ -3,10 +3,10 @@
 // would crash because it walked to the end of the upvalue list (correct), but
 // then didn't handle not finding the variable.
 
-new Fn {
+Fn.new {
   var a = "a"
   var b = "b"
-  new Fn {
+  Fn.new {
     IO.print(b) // expect: b
     IO.print(a) // expect: a
   }.call()