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/io/print_bad_to_string.wren

@@ -2,4 +2,4 @@ class BadToString {
   toString { 3 }
 }
 
-IO.print(new BadToString) // expect: [invalid toString]
+IO.print(BadToString.new()) // expect: [invalid toString]