src/language-handlebars/printer-glimmer.js/printOpenBlock
function printOpenBlock(path, print) {
  const node = path.getValue();

  const openingMustache = printOpeningBlockOpeningMustache(node);
  const closingMustache = printOpeningBlockClosingMustache(node);

  const attributes = [printPath(path, print)];

  const params = printParams(path, print);
  if (params) {
    attributes.push(line, params);
  }

  if (isNonEmptyArray(node.program.blockParams)) {
    const block = printBlockParams(node.program);
    attributes.push(line, block);
  }

  return group([
    openingMustache,
    indent(attributes),
    softline,
    closingMustache,
  ]);
}