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/core/fiber/resume_caller.wren

@@ -1,8 +1,8 @@
-var b = new Fiber {
+var b = Fiber.new {
   IO.print("fiber b")
 }
 
-var a = new Fiber {
+var a = Fiber.new {
   IO.print("begin fiber a")
   b.call()
   IO.print("end fiber a")