json: refine constraint for whitespace to avoid runaways yet allow pretty print (#7866)

examples/server/public/json-schema-to-grammar.mjs

@@ -1,5 +1,5 @@
 // WARNING: This file was ported from json_schema_to_grammar.py, please fix bugs / add features there first.
-const SPACE_RULE = '" "?';
+const SPACE_RULE = '| " " | "\\n" [ \\t]{0,20}';
 
 function _buildRepetition(itemRule, minItems, maxItems, opts={}) {
   if (minItems === 0 && maxItems === 1) {