Move TOC syntax highlighting into its own file
Move TOC settings to ftplugin/latextoc.vim

Also add a modeline for code style consistency.
Move TOC keybindings and assorted functions

The functions are moved into the ftplugin/latextoc.vim file because no
other function in the file uses them.
Make all TOC mappings nonrecursive

Just a good thing to do in case the user has some unexpected prior
mappings.