feat(vimtex): add vimtex

I spend a fair amount of time writing LaTeX at the moment, and IMO
vimtex just works really well

lua/plugins/spec/vimtex.lua

@@ -0,0 +1,12 @@
+--- @module 'lazy.nvim'
+--- @type LazySpec[]
+return {
+  "lervag/vimtex",
+  lazy = false,     -- we don't want to lazy load VimTeX
+  init = function()
+    -- VimTeX configuration goes here, e.g.
+	vim.g.tex_flavour='latex'
+	vim.g.vimtex_quickfix_mode=0
+    vim.g.vimtex_view_method = "skim"
+  end
+}