Add vertical nav bar support

[Vinicius-8]

Support for vertical nav bar, solving issue #95

Example:

GnavVertical.mp4

Implementation

For vertical mode, do not use the bottomNavigationBar. Instead, place the Gnav inside the body.
Set the axis to vertical by using GnavAxis.vertical in the axis property.

Code Example

import 'package:flutter/material.dart';
import 'package:google_nav_bar/google_nav_bar.dart';
import 'package:line_icons/line_icons.dart';

void main() => runApp(MaterialApp(
    builder: (context, child) {
      return Directionality(textDirection: TextDirection.ltr, child: child!);
    },
    title: 'GNav',
    theme: ThemeData(
      primaryColor: Colors.grey[800],
    ),
    home: Example()));

class Example extends StatefulWidget {
  @override
  _ExampleState createState() => _ExampleState();
}

class _ExampleState extends State<Example> {
  int _selectedIndex = 0;
  static const TextStyle optionStyle =
      TextStyle(fontSize: 30, fontWeight: FontWeight.w600);
  static const List<Widget> _widgetOptions = <Widget>[
    Text(
      'Home',
      style: optionStyle,
    ),
    Text(
      'Likes',
      style: optionStyle,
    ),
    Text(
      'Search',
      style: optionStyle,
    ),
    Text(
      'Profile',
      style: optionStyle,
    ),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        elevation: 20,
        title: const Text('GoogleNavBar'),
      ),
      body: Row(
        children: [
          Container(
            decoration: BoxDecoration(
              color: Colors.white,
              boxShadow: [
                BoxShadow(
                  blurRadius: 20,
                  color: Colors.black.withOpacity(.1),
                )
              ],
            ),
            child: SafeArea(
              child: SizedBox(  
                width: 100, // <--- Gnav width                    
                child: Padding(
                  padding: const EdgeInsets.symmetric(horizontal: 10, vertical: 8),
                  child: GNav(                
                    rippleColor: Colors.grey[300]!,
                    hoverColor: Colors.grey[100]!,
                    gap: 1,
                    activeColor: Colors.black,
                    iconSize: 24,
                    padding: EdgeInsets.symmetric(horizontal: 20, vertical: 12),
                    duration: Duration(milliseconds: 400),
                    tabBackgroundColor: Colors.grey[100]!,
                    color: Colors.black,
                    axis: GnavAxis.vertical, // <--- Define the axis to vertical
                    tabs: [
                      GButton(
                        icon: LineIcons.home,
                        text: 'Home',
                      ),
                      GButton(
                        icon: LineIcons.heart,
                        text: 'Likes',
                      ),
                      GButton(
                        icon: LineIcons.search,
                        text: 'Search',
                      ),
                      GButton(
                        icon: LineIcons.user,
                        text: 'Profile',
                      ),
                    ],
                    selectedIndex: _selectedIndex,
                    onTabChange: (index) {
                      setState(() {
                        _selectedIndex = index;
                      });
                    },
                  ),
                ),
              ),
            ),
          ),

          
          Expanded(
            child: Center(
              child: _widgetOptions.elementAt(_selectedIndex),
            ),
          ),
        ],
      ),
      
      // bottomNavigationBar: ,  <--- Don't use for vertical Nav Bar
    );
  }
}

closes #95