[decimal] Update Decimal constructors (#33)

This pull request includes several changes to the `DeciMojo` library to
simplify the initialization of `Decimal` objects and improve code
readability.

Codebase Simplification:
* Removed the `from_words` method and replaced its usage with direct
calls to the `Decimal` constructor.
* Added a new constructor to initialize `Decimal` objects with four raw
words and another constructor to initialize `Decimal` objects with five
components.

Testing:
* Imported the `testing` module to add assertions for validating the
`flags` word in the new constructor.
* Updated test cases to use the new `Decimal` constructor instead of
`from_words`.

Author: forfudan

README.md

@@ -2,7 +2,7 @@
 
 A fixed-point decimal arithmetic library implemented in [the Mojo programming language 🔥](https://www.modular.com/mojo).
 
-**[中文·漢字»](https://github.com/forFudan/DeciMojo/blob/main/docs/README_ZH.md)**
+**[中文·漢字»](./docs/readme_zht)**
 
 ## Overview