nvim/vimtex: set default tex flavor to LaTeX
nvim/vimtex: set default tex flavor to LaTeX

The vimtext plugin now requires having a default TeX flavor set (TeX,
ConTeXt or LaTeX), with the default set as TeX. Since I only work with
LaTeX, this commit adds the necessary config setting for nvim.

dplarson/dotfiles,dplarson/dotfiles