This commit adds support to add Lemmy RSS feeds to FeedDeck. A user can
provide the url of an Lemmy instance, the url of a community or of an
user.
The special thing of the Lemmy source in opposite to the normal RSS
source is, that we parse the provided link form a feed item, to check if
it contains a image, video or YouTube url, to apply some special
formatting.
This commit adds a new "Continuous Integration" GitHub Action, which is
used to run the tests for the Flutter app (and later also for Deno).
This commit also adds a first test, so that the GitHub Action does not
fail.
This PR also removes the Visual Studio Code and Neovim configurations
from the contribution guide, since I'm not using it anymore and for that
I do not want to maintain it any longer.
