Fix loading of js and css files

Due to some changes in server, the loading of files changed a bit and if
the output file is called main, core-main.js will be loaded instead.

This also move the js files to the src directory to standardize a bit
more with other Nextcloud apps

Signed-off-by: Carl Schwan <carl@carlschwan.eu>
Signed-off-by: nextcloud-command <nextcloud-command@users.noreply.github.com>

Author: CarlSchwan