Add support for one element lists

[dentrado]

Currently { "colors": ["#c33"] } results in the $colors variable being a just a color while this { "colors": ["#c33", "#c34"] } results in $colors being a list.

This can cause confusion eg. when iterating over a list of maps with @each as it will either pick the first element of the list or the first key-value pair in the first map depending on how many items the list has.

This PR adds support for one element lists by adding a trailing comma to all generated lists:

Comma-separated lists may have a trailing comma. This is especially useful because it allows you to represent a single-element list. For example, (1,) is a list containing 1 and (1 2 3,) is a comma-separated list containing a space-separated list containing 1, 2, and 3.

https://sass-lang.com/documentation/file.SASS_REFERENCE.html#lists

[lizell]

I like consistency!

[pmowrer]

Great call @dentrado, sorry for the delay!