Fetch items from 5etools

Regardless of how #8, we will need to fetch item data (and other data) from 5etools and convert it to a format we want. We need to do this in Typescript inside of the React app.

Our requirements:
- [ ] fetch `https://5etools-mirror-1.github.io/data/items-base.json`
- [ ] apply the following filters locally
- [ ] "Vanilla" sources
- [ ] no generic variants
- [ ] `map` the data to objects that match the `Item` interface in `ddtools-types`

This should be done in a new file `frontend/src/services/data.ts` in a function called `fetchItems` that can look like the following:

```ts
import { Item } from "ddtools-types";

export async function fetchItems(): Promise {
const url = "https://5etools-mirror-1.github.io/data/items-base.json";

const response = await window.fetch(url);
const data = await response.json(); // This will be the exact JSON data from 5etools with every item

// Filter out stuff from `data` and then map it to new objects in the `Item` format
const items = ???;

return items;
}
```

To accomplish this you'll have to familiarize yourself with:
- Typescript types and syntax
- Javascript promises and async/await
- Functional programming like using `map` on an array

Google these things or ask me about them. Good luck!