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/sequence/is_empty.wren

@@ -7,4 +7,4 @@ class InfiniteSequence is Sequence {
 }
 
 // Should not try to iterate the whole sequence.
-IO.print((new InfiniteSequence).isEmpty) // expect: false
+IO.print(InfiniteSequence.new().isEmpty) // expect: false