Add cquery support for c/c++ projects

Add cquery support for c/c++ projects

Implements minimal support for the core cquery language
server. None of its extensions are implemented yet.

* eglot.el (eglot-server-programs): Add cquery to list
of guessed programs for c-mode and c++-mode.
(eglot-initialization-options eglot-cquery): Specialize init
options to pass cquery a cache directory and disable a flood
of $cquery/progress messages.
(eglot-handle-notification $cquery/publishSemanticHighlighting):
(eglot-handle-notification $cquery/setInactiveRegions):
(eglot-handle-notification $cquery/progress): New no-op functions
to avoid filling logs with "unknown message" warnings.
(eglot-cquery): New eglot-lsp-server subclass.

* README.md: Mention cquery in the README.