Add support for cmake-language-server

[nospam2998]

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)

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.