Add devcontainer configuration for vscode

[jbrazio]

This pull request introduces the configuration for a basic functional MeshCore workspace, which could serve as a foundational for further standardisation (such as code formatting, settings, etc.) down the line, if needed. Devcontainers offer a stable and consistent development environment for multiple users; this pull request has no impact on non-VS Code users or those who choose not to use them.

[4np]

It would be good to have the README.md extended (or a new dedicated document drafted) to explain in clear steps exactly how to work on the code, to debug the firmware, etcetera. It would make it easier for more people to contribute to the firmware.

For example, I have been doing this on macOS:

• Install Miniconda or Miniforge (conda is a Python environment manager):

#curl -fsSLo /tmp/install.sh https://repo.anaconda.com/miniconda/Miniconda3-latest-MacOSX-arm64.sh
curl -fsSLo /tmp/install.sh "https://github.com/conda-forge/miniforge/releases/latest/download/Miniforge3-MacOSX-$(uname -m).sh"
/usr/bin/env zsh "/tmp/install.sh" -b -f -p "$HOME/.miniforge"

• Add it to your path:

export PATH="$PATH:$HOME/.miniforge/bin"

• Install PlatformIO:

pip install --upgrade platformio

• Clone the repo and switch to the dev branch:

git clone https://github.com/meshcore-dev/MeshCore.git
git checkout dev

• AdHoc build of RAK4631 BLE firmware:

sh build.sh build-firmware RAK_4631_companion_radio_ble
cp .pio/build/RAK_4631_companion_radio_ble/firmware.zip ~/Downloads/RAK_4631_companion_radio_ble-adhoc-$now-firmware.zip
cp .pio/build/RAK_4631_companion_radio_ble/firmware.uf2 ~/Downloads/RAK_4631_companion_radio_ble-adhoc-$now-firmware.uf2

But it would be good to explain how to use PlatformIO inside VSCode and to build, run and debug the firmware to standardize the development process even more.

[jbrazio]

This is exactly the use where dev containers shine.
Every developer gets exactly the same build environment no matter the underlaying OS without any need to install packages or even platformio on the host system.