Add <Menu.ResourceItem>component to facilitate menu customization

[fzaninotto]

Problem

Customizing the default menu items leads to boilerplate and copy paste

Solution

Introduce a dedicated component for resource menu items.

Before

import { Menu } from 'react-admin';

import BookIcon from '@mui/icons-material/Book';
import ChatBubbleIcon from '@mui/icons-material/ChatBubble';
import PeopleIcon from '@mui/icons-material/People';
import LabelIcon from '@mui/icons-material/Label';

export const MyMenu = () => (
    <Menu>
        <Menu.DashboardItem />
        <Menu.Item to="/posts" primaryText="Posts" leftIcon={<BookIcon />}/>
        <Menu.Item to="/comments" primaryText="Comments" leftIcon={<ChatBubbleIcon />}/>
        <Menu.Item to="/users" primaryText="Users" leftIcon={<PeopleIcon />}/>
        <Menu.Item to="/custom-route" primaryText="Miscellaneous" leftIcon={<LabelIcon />}/>
    </Menu>
);

After

import { Menu } from 'react-admin';
import LabelIcon from '@mui/icons-material/Label';

export const MyMenu = () => (
    <Menu>
        <Menu.DashboardItem />
        <Menu.ResourceItem name="posts" />
        <Menu.ResourceItem name="comments" >
        <Menu.ResourceItem name="users" />
        <Menu.Item to="/custom-route" primaryText="Miscellaneous" leftIcon={<LabelIcon />}/>
    </Menu>
);