Install 'code' from snap store or any other method.
You'll be installing a bunch of packages.
sudo apt install g++
sudo apt install cmake
sudo apt install git
sudo apt install libgl1-mesa-dev libxss-dev libxxf86vm-dev libxkbfile-dev libxv-dev
sudo apt install libxrandr-dev libxinerama-dev libxcursor-dev libxi-dev
In vs code install the "C/C++ Extension Pack" from the store and then open the project folder and run:
git submodule init and git submodule update
Finally Ctrl-Shift-P and run Cmake: Configure and then do it again and run Cmake: Build and it should work.
One of these days pebble will learn how to deploy a build for convenience.