Add support for extend primop II

grammar.js

@@ -463,9 +463,15 @@ module.exports = grammar({
     //grammar.lalrpop: 509
     // Different from lalrpop grammar, we parse all possible builtins, not just
     // the defined ones.
-    builtin: $ => seq(
+    builtin: _ => seq(
       "%",
-      $.ident,
+      // We are a bit more liberal with what can go in a builtin function than
+      // for identifiers, because builtins are properly delimited by `%`.
+      // Upstream Nickel added `/` as a valid character already, so there's a
+      // precendent for extensions (although it's not very likely), so we try to
+      // be a bit future-proof. We just make sure the builtin starts with either
+      // a letter or an undescore, to ensure reasonable names.
+      /_*[a-zA-Z][a-zA-Z0-9./_'-]*/,
       "%",
     ),
 

src/grammar.json

@@ -1909,8 +1909,8 @@
           "value": "%"
         },
         {
-          "type": "SYMBOL",
-          "name": "ident"
+          "type": "PATTERN",
+          "value": "_*[a-zA-Z][a-zA-Z0-9./_'-]*"
         },
         {
           "type": "STRING",

src/node-types.json

@@ -239,17 +239,7 @@
   {
     "type": "builtin",
     "named": true,
-    "fields": {},
-    "children": {
-      "multiple": false,
-      "required": true,
-      "types": [
-        {
-          "type": "ident",
-          "named": true
-        }
-      ]
-    }
+    "fields": {}
   },
   {
     "type": "chunk_expr",