generate config files on build time

[milahu]

on build time, run this to write all config files to app/config/linux/

# enable the original install function in native-autoinstall.js
export VDHCOAPP_INSTALL_ON_BUILDTIME=1

echo generating config files for linux
export HOME=$PWD/app/config/linux
grep '^only_if_dir_exists = "~/\.' config.toml | cut -d'"' -f2 |
  while read d; do mkdir -p "$HOME/${d:2}"; done
./app/src/main.js install

on runtime, run vdhcoapp install
to create symlinks from $HOME to the installed app/config/linux/

example

$ readlink /home/user/.config/chromium/NativeMessagingHosts/net.downloadhelper.coapp.json
/nix/store/0nrvqzclq34a53nyk89h7b4i3dq9mdyr-vdhcoapp-2.0.19/opt/vdhcoapp/config/linux/.config/chromium/NativeMessagingHosts/net.downloadhelper.coapp.json

im using this in vdhcoapp.nix
where all paths are constant so the generated config files are always identical
and when running vdhcoapp install again on runtime
it is cheaper to compare symlinks
instead of generating the config files and comparing file contents

where all paths are constant

this will be true only for some users
so probably this will not be merged into the main branch

based on #217