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/break/closure_in_for.wren

@@ -1,7 +1,7 @@
 var f
 for (i in [1, 2, 3]) {
   var j = 4
-  f = new Fn { IO.print(i + j) }
+  f = Fn.new { IO.print(i + j) }
   break
 }
 

test/language/break/closure_in_while.wren

@@ -1,7 +1,7 @@
 var f
 while (true) {
   var i = "i"
-  f = new Fn { IO.print(i) }
+  f = Fn.new { IO.print(i) }
   break
 }
 

test/language/break/in_function_in_loop.wren

@@ -1,6 +1,6 @@
 var done = false
 while (!done) {
-  new Fn {
+  Fn.new {
     break // expect error
   }
   done = true