static findFunctionDeclarationNode(name, ast) {
    if (!name) return null;

    for (const node of ast.program.body) {
      if (node.type === 'FunctionDeclaration' && node.id.name === name) return node;
    }

    return null;
  }
find FunctionDeclaration node.
@param {string} name - function name.
@param {AST} ast - find in this ast.
@returns {ASTNode|null} found ast node.