Add a command to open a new tab with specific level

[piroor]

piroor/treestyletab#2753

Assume that there is a tree:

• [Group/Work]
  • A
    • A1 (active)
  • B
• [Group/Private]
  • C

The proposal command is: open a new tab D as a last child of "[Group/Work]" directly, like:

• [Group/Work]
  • A
    • A1 (active)
  • B
  • D (new tab)
• [Group/Private]
  • C

[piroor]

Note: I have less motivation to implement new features by my hand, and pull requests to implement "good first issue" labeled features like this are always welcome. I hope these issues get people involved to join opensource development.

A new keyboard shortcut will be added here:

tst-more-tree-commands/manifest.json

Lines 48 to 49 in c18b790

	}
	},

tst-more-tree-commands/background/background.js

Lines 354 to 355 in c18b790

	return;
	}

Codes for a new menu command will be added here:

tst-more-tree-commands/background/background.js

Lines 155 to 156 in c18b790

	}
	};

tst-more-tree-commands/background/background.js

Lines 225 to 226 in c18b790

	default:

tst-more-tree-commands/background/background.js

Lines 284 to 285 in c18b790

	default: