feat: adapt to changes in telescope api 0.1.6

[mgierada]

Fixes the issue with launching git-worktree.nvim with telescope v. 0.1.6

stack traceback:
	[C]: in function 'nvim_win_get_width'
	...l/share/nvim/lazy/telescope.nvim/lua/telescope/utils.lua:287: in function 'calc_result_length'
	...l/share/nvim/lazy/telescope.nvim/lua/telescope/utils.lua:347: in function 'transform_path'
	...worktree.nvim/lua/telescope/_extensions/git_worktree.lua:166: in function 'parse_line'
	...worktree.nvim/lua/telescope/_extensions/git_worktree.lua:179: in function 'git_worktrees'
	/Users/maciej/.config/nvim/lua/mappings.lua:77: in function </Users/maciej/.config/nvim/lua/mappings.lua:76>

while running with telescope 0.1.6.

Also, adding doc strings and running some formatting

Fixes #122 and related to #123