Add ccls and vscode-ccls extension

[kammoh]

Please consider adding vscode-ccls to the list of VS Code C/C++/Objective-C extensions.

ccls is a is a C/C++/Objective-C language server originating from cquery.
vscode-ccls is the VS Code extension which provides ccls services.
vscode-ccls seems to be the only working extension which provides C/C++ refactoring.