Code Refactored for Improved Quality, Readability, and Maintainability

[Yash-javiya]

Refactored code using this techniques:

1. Rename method/variable: changes in simplesqlite and test package

2. Introduce explaining variable: changes n test/test_simplesqlite.py file

3. Extract method: In simplesqlite/core.py and simplesqlite/_validator.py file

4. Move method/field: moved extract_table_metadata method to simplesqlite/core.py file and did the necessary changes in simplesqlite/_func.py file.