User variable

[Maingron]

See #60 (system variable), but for storing user data exclusively.
When logging into a user (or maybe fetching user list), we then can combine this variable with the system variable.
Example contents of user variable:

{
  "name": "sysacc",
  "person": {
    "name": "System",
    "surname": "Account",
    "birthdate": "2000-12-31",
    "gender": "",
  },
  "paths": {
    "home": system.paths.users + user.name + "/",
    "appdata": user.paths.home + "appdata/",
    "desktop": user.paths.home + "desktop/",
    "images": user.paths.home + "images/",
    "savestates": user.paths.home + "saved games/"
  },
  "settings": {
    "themecolor": "#ffff77",
    "fontsize": "16px",
    "fontfamily": "Comic Sans MS",
    "darkmode": "auto",
    "developer": false
  }
}
// And so on