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/function/arity.wren

@@ -1,5 +1,5 @@
-IO.print(new Fn {}.arity) // expect: 0
-IO.print(new Fn {|a| a}.arity) // expect: 1
-IO.print(new Fn {|a, b| a}.arity) // expect: 2
-IO.print(new Fn {|a, b, c| a}.arity) // expect: 3
-IO.print(new Fn {|a, b, c, d| a}.arity) // expect: 4
+IO.print(Fn.new {}.arity) // expect: 0
+IO.print(Fn.new {|a| a}.arity) // expect: 1
+IO.print(Fn.new {|a, b| a}.arity) // expect: 2
+IO.print(Fn.new {|a, b, c| a}.arity) // expect: 3
+IO.print(Fn.new {|a, b, c, d| a}.arity) // expect: 4