function isFunction(node) {
	return Boolean(node && anyFunctionPattern.test(node.type));
}
Checks whether a given node is a function node or not.
The following types are function nodes:

- ArrowFunctionExpression
- FunctionDeclaration
- FunctionExpression
@param {ASTNode|null} node A node to check.
@returns {boolean} `true` if the node is a function node.