Add support for cmake-language-server

**Name:** cmake-language-server
**URL:** https://pypi.org/project/cmake-language-server/

This language server for CMake files comes with the following features (quoted from its own description):

* Builtin command completion
* Documentation for commands and variables on hover
* Formatting (by [cmake-format](https://github.com/cheshirekow/cmake_format))

I.e. not much in addition to what ale already handles by interacting with cmake-format by itself. At least not enough for me to bother even attempting to use it. Still filing an issue, in case someone else might have a greater desire for command completion, or if the language server becomes more competent in the future.