Add support for ltex-ls (latex grammar checking)

**Name:** ltex-ls
**URL:** https://valentjn.github.io/ltex/, (code at https://github.com/valentjn/ltex-ls)

> LTEX LS (LTEX Language Server) implements a language server according to the Language Server Protocol (LSP) and provides grammar and spelling errors in markup documents (LATEX, Markdown, etc.). The documents are checked with LanguageTool.

At least the vscode client for ltex-ls seems to work pretty well. I haven't found a spell and grammar checking tool for latex that works well for me, but this one looks promising. There are also some neovim plugins using the built-in LSP support ([here is a list of client editor plugins](https://valentjn.github.io/ltex/installation-usage.html)), but I don't use that so ALE integration would very nice :)