Please expose `extname`

[aMarCruz]

Hi team,
It is common practice to include an additional filter for extensions, exposing extname avoids dependency on node's path.

But the current extname behavior in pluginutils is different from the node version when the name starts or ends with a dot:

filename	node	pluginutils
.file	''	'.file'
/.file	''	'.file'
./.file	''	'.file'
file.	'.'	''
./file.	'.'	''

this behave like in node, and do not depends in basename:

function extname ( path ) {
  const match = /(?!\.\/\\|\.\.)[^/\\](\.[^./\\]*)$/.exec( path );
  return match ? match[1] : '';
}

I can submit a PR if you wish.

[aMarCruz]

oh, not an useful export, pluginutils depends on node's path anyway, so I think addExtension should use the node version.

[Rich-Harris]

Thanks, sorry for slow response – this is fixed in 2.0.1 via 0e46f90