Allow coc to see avr include directories

[K4LCIFER]

Is your feature request related to a problem? Please describe.
Coc doesn't recognize #include <avr/io.h> for example. Leading to it not recognizing any code used from those header files.

Describe the solution you'd like
I would like for coc to be able to see these directories. I believe that they are all located in /usr/avr/include/avr

[oblitum]

You have to configure your language server flags.

[K4LCIFER]

You have to configure your language server flags.

How would I go about doing that to accomplish what I'm looking for?

[oblitum]

See its docs, for clangd for example: https://clangd.llvm.org/guides/system-headers.