[Feature] `conf.user.json` allows to change the theme and the way images are inserted

[egeres]

According to this documentation: https://support.typora.io/Advance-Config/, the current conf.user.json does not allow to change the theme (The Typora arguments also won't allow this! But that would see like too much for me https://support.typora.io/Launch-Arguments/). On top of this, the way images are inserted (if uploaded, copied to a custom folder, copied to ./assets, if rules are applied to local images etc...) is also not exposed in the user configuration file

In my view, exposing these configuration parameters to a .json (like what VSCode does with it's settings.json) allows for automating the configuration with scripts and what not

[egeres]

Actually, the general version of this issue is to allow users to change everything from the configuration from the conf.user.json file, I'm not sure about how difficult would it be to do such a refactor or if there are plans to do this in the future