feat: add 'function' support to fp-sdk's primitive type guards

[nodefect]

Adds Function to the set of supported primitive types in Dyn.cast().to() and Dyn.tryCast().to().

Changes

• Extended the conditional return types in src/dyn.ts to include FunctionConstructor → Function
• Added test coverage for function casting in src/fpSdk.test.ts

Why

fp-sdk already supports dynamic casting for primitive types (String, Number, Boolean) and custom classes, but Function was missing from the type mapping and had no test coverage. This PR fills that gap so users can now do:

const fn = () => "hello";
Dyn.tryCast(fn).to(Function); // Option<Function>