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.wren

@@ -3,7 +3,7 @@ class Foo {
 }
 
 // Calls toString on argument.
-IO.print(new Foo) // expect: Foo.toString
+IO.print(Foo.new()) // expect: Foo.toString
 
 // With one argument, returns the argument.
 IO.print(IO.print(1) == 1) // expect: 1

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]

test/io/write_bad_to_string.wren

@@ -2,5 +2,5 @@ class BadToString {
   toString { 3 }
 }
 
-IO.write(new BadToString) // expect: [invalid toString]
+IO.write(BadToString.new()) // expect: [invalid toString]
 IO.print