consider clangd? much better than MS's

[asukaminato0721]

https://github.com/clangd/clangd

[guyutongxue]

We want a folder-level config for user to choose whether use cpp-tools or clangd. Since clangd cannot be configured as disabled by extension setting, user must manually disable or uninstall the extension to prevent conflict (if he wants to use cpp-tools in some specific folder), which is not achiveable in a programmatically pre-configured way. See #26.