Add code coverate tool

.gitignore

···
 .direnv
 .envrc
 .vscode
+tarpaulin-report.html

nix/devshell.nix

···
         commands = [
           {
             name = "book";
+            help = "Build book at book/book";
             command = "mdbook build book";
+          }
+
+          {
+            name = "coverage";
+            help = "Test coverate at tarpaulin-report.html";
+            command = "cargo tarpaulin --out Html";
           }
         ];
 
···
           [
             pkgs.cargo
             pkgs.rust-analyzer
+            pkgs.cargo-tarpaulin
             pkgs.mdbook
             zigcc
           ];