src/language-html/print-preprocess.js/removeIgnorableFirstLf
function removeIgnorableFirstLf(ast /*, options */) {
  ast.walk((node) => {
    if (
      node.type === "element" &&
      node.tagDefinition.ignoreFirstLf &&
      node.children.length > 0 &&
      node.children[0].type === "text" &&
      node.children[0].value[0] === "\n"
    ) {
      const text = node.children[0];
      if (text.value.length === 1) {
        node.removeChild(text);
      } else {
        text.value = text.value.slice(1);
      }
    }
  });
}