Create models/Item.ts

[davdwan21]

Describe your changes

Changes:

• Made a few assumptions when creating the ItemSchema since there was only one example
• Created two nested Schemas for threshold and notifications for readability
• Exported types in a similar manner to models/Product.ts

Next steps:

• Create services/items.ts to connect to database and convert info to objects
• Update enums and schemas to match more details specs (assumed to be provided later)

Issue ticket number and link

[ Insert Link & Ticket #]

Checklist before requesting a review

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• This change requires a documentation update

Extra Notes

I'm not sure if my itemSchema is robust, since only one example Schema is provided. Let me know if I need to adjust any of the assumptions I made.