Improve detection of string concatenation

[thecrypticace]

Previously we'd check to see if a string literal was immediately nested inside a concat expression, for example:

export function Example() {
  return <div className={' flex ' + ' border ' + ' sm:hidden '} />
}

However, this breaks down when the strings being sorted are nested inside other expressions. For example, a ternary expression:

export function Example() {
  return <div class={'flex' + (standalone ? ' border ' : 'outline) + ' sm:hidden'}></div>
}

The way this works is if a string is nested inside a concat expression at any level then we'll do our best to keep the relevant whitespace. Obviously, due to runtime evaluation, we can't guarantee that all necessary whtiespace will be preserved, or that all unnecessary whitespace will be removed, but this improves the situation for the vast majority of cases.

Fixes #285

This depends on #287 and that PR must be merged first