Configure cmake-language-server and coc-cmake

Utilises the following Python package:

```console
$ pip3 install --user cmake-language-server
```

autoload/build.vim

@@ -40,6 +40,7 @@ function! build#dir(...) abort
     " TODO: Check .vim/coc-settings.json exists, create it if not
     let l:coc_settings = json_decode(join(readfile('.vim/coc-settings.json'), ''))
     let l:coc_settings['clangd.compilationDatabasePath'] = $BUILD_DIR
+    let l:coc_settings['cmake.lsp.buildDirectory'] = $BUILD_DIR
     call writefile([json_encode(l:coc_settings)], '.vim/coc-settings.json')
     CocRestart
   endif