[node-build-scripts] fix(css-variables): preserve string quotes

[adidahiya]

Changes proposed in this pull request:

Before this PR, strings inside Sass lists would lose their quotes during the generate-css-variables build step.

The following source code ...

// src/common/_variables.scss
$pt-font-family: -apple-system,
                 "BlinkMacSystemFont",
                 "Segoe UI",
                 "Roboto",
                 "Oxygen",
                 "Ubuntu",
                 "Cantarell",
                 "Open Sans",
                 "Helvetica Neue",
                 $icons16-family, // support inline Blueprint icons
                 sans-serif !default;

... would become ...

// lib/scss/variables.scss
$pt-font-family: -apple-system, BlinkMacSystemFont, Segoe UI, Roboto, Oxygen,
  Ubuntu, Cantarell, Open Sans, Helvetica Neue, blueprint-icons-16, sans-serif !default;

This created some problems for strings containing any characters which would confuse the Sass syntax parser, such as ../ (used in file path strings, not in this repo but in another repo which uses @blueprintjs/node-build-scripts).

To work around this problem and preserve the quotes for those strings, I've added some hacky logic to detect quoted strings in generate-css-variables, and wrap those values with quotes before writing the output to variables.scss.

After this PR, we get:

// lib/scss/variables.scss
$pt-font-family: -apple-system, "BlinkMacSystemFont", "Segoe UI", "Roboto",
  "Oxygen", "Ubuntu", "Cantarell", "Open Sans", "Helvetica Neue",
  "blueprint-icons-16", sans-serif !default;

Note that I limited the special case code added here to apply just those Sass lists which contain , comma delimiters. This was done to avoid acting on lists of values which make up a compound CSS property like box-shadow, e.g. inset 0 0 0 1px rgba($black, 0.2) -- Sass considers those to be lists as well, but the values inside those "lists" should not be treated as strings.

Reviewers should focus on:

No regressions in other parts of lib/scss/variables.scss