feat: add flake.nix (#1843)

* dev: add flake.nix

* feat: init dev, unstable, and nixvim config

* docs: update description

* dev: split neovim with nixvim

* feat: init tinymist server

* x

* g

* g

docs/tinymist/nix.typ

@@ -0,0 +1,28 @@
+
+= System Configuration
+
+Add following content to the end of your `~/.config/nix/nix.conf` file to enable flakes and nix-command experimental-features:
+
+```conf
+experimental-features = nix-command flakes
+```
+
+To debug bundle size:
+
+```
+nix-shell -p nix-tree.out --run nix-tree
+```
+
+= Running devShell
+
+Run with built binaries from source:
+
+```
+nix develop
+```
+
+Run with the binaries from the nixpkgs unstable:
+
+```
+nix develop .#unstable
+```