🐛 Re-add trailing path delimiter to RegExp

I had removed it in obahareth#13 (sorry, I had been struck by yarnpkg/yarn#4722)

The behaviour _with_ the trailing path delimiter is more correct, as without it in theory modules starting with the same name as a non-ES5 node_module would be unneccessarily transpiled.

This is all assuming the leading and trailing slash before obahareth#13 were meant as path delimiters and not RegExp delimiters, meaning that anyone wanting to use the RegExp would (and will) have to add surrounding slashes or `new RegExp()` manually, as in `/<place-output-here>/` or `new RegExp('<place-output-here>')` (personally, I'm writing the output of _are-you-es5_ to a file on _postinstall_ and read it into a `new RegExp()` in the webpack config).

src/babel-loader-regex-builder.ts

@@ -1,3 +1,3 @@
 export function getBabelLoaderIgnoreRegex(dependencies: string[]) {
-  return `[\\/]node_modules[\\/](?!(${dependencies.join('|')}))`
+  return `[\\/]node_modules[\\/](?!(${dependencies.join('|')}))[\\/]`
 }