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/benchmark/fibers.wren

@@ -5,7 +5,7 @@ var sum = 0
 var start = IO.clock
 
 for (i in 0...100000) {
-  fibers.add(new Fiber {
+  fibers.add(Fiber.new {
     sum = sum + i
     if (i < 99999) fibers[i + 1].call()
   })