Use the following include paths for intellisense completion:
"${env:PROS_TOOLCHAIN}/arm-none-eabi/include", "${env:PROS_TOOLCHAIN}/lib/gcc/arm-none-eabi/6.3.1/include", "${workspaceRoot}", "${workspaceRoot}/include"
Add "_INTELLISENSE" to your defines to fix certain intellisense errors that will actually compile without issue