Include groups in list exporting

[yorkxin]

Summary

• When exporting list of tabs, if tabs are grouped, the Markdown code will also be grouped by tab groups along with their names.
• Tab Grouping is available on Google Chrome and Microsoft Edge. On browsers that don't support this feature such as Firefox, a flattened list will be generated, as it used to be.

Example:

- [Page 0 - Copy as Markdown](http://localhost:5566/0.html)
- Group 1
  - [Page 1 - Copy as Markdown](http://localhost:5566/1.html)
  - [Page 2 - Copy as Markdown](http://localhost:5566/2.html)
- [Page 3 - Copy as Markdown](http://localhost:5566/3.html)
- Untitled green group
  - [Page 4 - Copy as Markdown](http://localhost:5566/4.html)
- [Page 5 - Copy as Markdown](http://localhost:5566/5.html)

Where Group 1 is the custom name of the group, and Untitled green group is a group that has never been named by the user.

For Task List, the group will also become a task:

- [ ] [Page 0 - Copy as Markdown](http://localhost:5566/0.html)
- [ ] Group 1
  - [ ] [Page 1 - Copy as Markdown](http://localhost:5566/1.html)
  - [ ] [Page 2 - Copy as Markdown](http://localhost:5566/2.html)
- [ ] [Page 3 - Copy as Markdown](http://localhost:5566/3.html)
- [ ] Untitled green group
  - [ ] [Page 4 - Copy as Markdown](http://localhost:5566/4.html)
- [ ] [Page 5 - Copy as Markdown](http://localhost:5566/5.html)

Closes #123

Tests

• [x] Chrome stable (macOS)
• [ ] Firefox stable (macOS)
• [ ] Chrome stable (Windows)
• [ ] Firefox stable (Windows)

Optional:

• [ ] Chrome beta (macOS)
• [ ] Firefox beta (macOS)
• [ ] Chrome beta (Windows)
• [ ] Firefox beta (Windows)