Rework Nix example project & add Nix template

[jaschutte]

Nix changes

Changes the Nix infrastructure inside of simple-nix to use Nix flakes. Adds some more explanation how to include dependencies using Nix and includes a Nix template. All of this was done to make it more user friendly (...for Nix users) to get started with Clash without having to have very deep knowledge of Nix or how Cabal works to make Nix and Cabal work together happily.

People who have Nix installed can now use immediately start using Clash with the simple-nix directory by running three commands:

mkdir my-project
cd my-project
nix flake init -t github:clash-lang/clash-starters

After this nix command is executed, it will display a 'welcome' message which gives a plain overview of what the Nix related commands do and instructs the user to read the README.md for more information.

Template changes

Aside from the Nix specific changes, we changed the entire way templates are set up. Nix specific templates can now be placed under the projects-nix sub-directory. This was an essential change to make as Nix and Stack templates are not compatible with each other. Nix and Stack in general do not play very nicely together so dropping Stack support for the Nix related template does not lose much. Files under clash-starters-files are now also copied verbatim into the clash-starters directory.

Original PR

The original PR is: clash-lang/clash-starters#6
Which I have closed after realizing that repository is essentially just a mirror

Credits

Big credits to @DigitalBrains1 for his massive help regarding everything really and @rslawson for testing!

[DigitalBrains1]

So there is a pretty major change involved with this. Before this PR, all our starter projects use the latest released version of Clash. With this PR, the Nix starter suddenly starts using Clash master. I don't think we should do that. I think it should keep on using the latest released version, and if currently our clash-compiler repo is not set up to support that (I don't know whether this is the case), we should make it such that it is.

If we decide to switch Nix users to Clash master (you somehow manage to convince me :-), then we should thoroughly document this in these starter projects as it is surely surprising.

[jaschutte]

Alright! This should use version 1.8.4 of clash now :)

One notable change is that I renamed {{name}}.cabal to simple-nix.cabal. This is because it also contains a Nix template, which does not substitute the {{name}} with the project name (and thus it all crashes and burns).

Short list of the down/upside of this change:
- have to manually rename the cabal
+ get an easy to use Nix template in return

[jaschutte]

I have no clue why Github keeps closing this PR automatically???

Anyways, a lot of things in this PR changed regarding the Nix/Stack template side of things. simple-nix is now under a different directory for Nix related templates and no longer supports stack as an option.

The reasoning for this is as follows:

• Nix and Stack templates were incompatible
• If you were using Nix, there is no reason to use Stack (as Nix handles the dependencies and uses Cabal over stack)

[DigitalBrains1]

Excellent, I think we're there. Just one teeeeensy little edit...