Moved `scss_maps` template from map of all sprites to map variable per sprite

As planned in #29, we are moving from all sprites in a `$sprite` variable to a variable per sprite. This PR introduces that change. In this PR:

- Update template to use `${{name}}` for each sprite
- Remove `sprite` function as it is now redundant to `map-get`
- Updated expected test template output
- Updated tests to use `${{name}}` for CSS validation

@leevigraham @makeable Would you mind approving this PR?