major: Switch to forward slashes(+more) to fix compilation on Linux (#1403)

Notably also adds some metadata files for NixOS 

* add support for linux clang cross compiles

* add linux clang instructions

* un-capitalize Mob.horse.*

* update the description in flake.nix

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Co-authored-by: Loki <lokirautio@gmail.com>

.gitignore

@@ -414,3 +414,9 @@ build/
 tmp*/
 _server_asset_probe/
 server-data/
+
+# Nix
+result
+result-*
+.direnv/
+.xwin-cache/