Handle balanced parens when searching for helper functions

[thecrypticace]

Fixes #1285

The regex would cause things like this to not work properly:

.foo {
  color: my-config(theme('fontFamily.sans'))
}

You'd get an error message like this:

'theme('fontFamily.sans' does not exist in your theme config.(invalidConfigPath)

Which means:

1. It saw config instead of my-config
2. It didn't handle nested parens correctly
3. the nested theme function wasn't found

This PR fixes all three of these