rename nixshell -> nix_shell

tree-sitter doesn't allow hyphens in the grammar name
underscore is the approach used by e.g. tree-sitter-c-sharp

grammar.js

@@ -1,5 +1,5 @@
 module.exports = grammar({
-  name: 'nixshell',
+  name: 'nix_shell',
 
   rules: {
     // TODO: allow trailing whitespace on any line