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.

doc/site/index.markdown

@@ -14,7 +14,7 @@ a familiar, modern [syntax][].
       }
     }
 
-    var adjectives = new Fiber {
+    var adjectives = Fiber.new {
       ["small", "clean", "fast"].each {|word| Fiber.yield(word) }
     }