[node-build-scripts] fix(css-variables): string quotes for all values

[adidahiya]

Changes proposed in this pull request:

#5303 fixed generate-css-variables to preserve string quotes for values inside lists, but after merging that I realized it doesn't actually target the broken use case I needed it to, which is a simple Sass variable value of a single string, like this:

// src/common/_variables.scss
$resources-path: "../../resources" !default;

Turns out we can simplify the change made in #5303 and apply it to all values which are not (List, Map, Color, or Number) in the JSON encoder.

So now we expect the above variable to be preserved exactly as-is in the output:

// lib/scss/variables.scss
$resources-path: "../../resources" !default;

Reviewers should focus on:

No regressions w/r/t #5303