Added g:vimtex_index_split_width
Added g:vimtex_index_split_width

This variable is the same as the previous g:vimtex_toc_width.

Also changed 'vimtex_index_split' to 'vimtex_index_split_pos'.

kmarius/vimtex,lervag/vimtex,lervag/vimtex,Aster89/vimtex,Aster89/vimtex,kmarius/vimtex