This PR introduces a Nix flake, allowing for InfiniTime to be built as a
Flake, including a FHS development environment.
It's derived from #1850 and
c57c57f3c6.
We also introduce `flake-compat`, allowing for non-Flake Nix mahcines to
use the project as-is, both for building (`default.nix`), and
development (`shell.nix`).
Additionally, we introduce `.envrc`, meaning that with `direnv`, the Nix
Flake is activated automatically.
Fixes#1850.
Signed-off-by: Dom Rodriguez <shymega@shymega.org.uk>
