Also add `.envrc` for direnv + Nix users. This makes developing locally
easier for us NixOS folks.

The flake itself will allow NixOS users to pull code directly from
Conduit's repository, making it completely trivial to stay up-to-date
with every commit.

I'd also like to add a NixOS module directly to this repository at some
point so that new configuration options will be available in the NixOS
module faster. But for now, NixOS users can simply override
`serivces.matrix-conduit.package` and get pretty much all the
functionality.

I've added myself to the `CODEOWNERS` file for the Nix files, since I am
willing to maintain this stuff. I use Conduit on NixOS so I'm personally
invested in having this work.

Lastly, `.gitignore` was updated to exclude symlinks created by `direnv`
and `nix build` and other such Nix commands.

This doesn't come without maintenance burden, however:

* The `sha256` in `flake.nix` will need to be updated whenever Conduit's
  MSRV is updated, but that should be pretty infrequent.

* `nix flake update` should be run every so often to pull in updates to
  `nixpkgs` and other flake inputs. I think downstream users can also
  override this themselves with `inputs.<name>.inputs.<name>.follows`.

* `nix flake check` should be run in CI to ensure Nix builds keep
  working.

* `nixpkgs-fmt --check $(fd '\.nix')` (or similar) should be run in CI
  to ensure style uniformity.