Meta: Add configuration file for YouCompleteMe

[gunnarbeutner]

YouCompleteMe is a plugin for Vim which provides code-completion functionality. This change adds a configuration file which makes YouCompleteMe aware of which compile flags to use with clangd.

https://user-images.githubusercontent.com/388571/197971914-7a05979b-9f0f-489e-b099-589cc689a516.mp4

[gunnarbeutner]

Unfortunately that wouldn't work because YCM tries to find its configuration file in the directory that contains the file that's being edited - and its parent directories.

g:ycm_extra_conf_globlist only whitelists certain config files because these files can contain arbitrary Python code.

There is a setting g:ycm_global_ycm_extra_conf which lets you specify the path for the config file but this would also take effect when editing source files outside of the serenity source directory.