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/language/method/arity.wren

@@ -19,7 +19,7 @@ class Foo {
   method(a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p) { a + b + c + d + e + f + g + h + i + j + k + l + m + n + o + p}
 }
 
-var foo = new Foo
+var foo = Foo.new()
 IO.print(foo.method) // expect: getter
 IO.print(foo.method()) // expect: no args
 IO.print(foo.method(1)) // expect: 1