[core,colors] fix !default modifiers in Sass variables

[adidahiya]

Changes proposed in this pull request:

Fixes the Sass variables issues I described in #5225 (comment).

The ! default modifier was accidentally removed from Sass variables in #4941. Adding back the CLI argument brings those back, both for core and colors packages.

Also I've removed the ability to override $ns, as it is meaningless to do so.

[blueprint-bot]

[core,colors] fix !default modifiers in Sass variables

Previews: documentation | landing | table | demo

[bradchristensen]

I don't think this has had the intended effect - the output files (lib/less/colors.less and lib/less/colors.scss) are still being generated without the !default flags.

I started digging and it looks like the getSassVarsSync method being used in the generate-css-variables script is just throwing away those flags, even though they're being retained first by the retainDefault flag passed into the script. cleanedInput is correct, but parsedVars is not. This actually seems to match up with the example on the get-sass-vars README where it shows the !default flag being thrown away there too: https://github.com/niksy/get-sass-vars#indexscss

It looks like this might have only become an issue quite recently in #5246 when the script was rewritten for interop with node-sass and dart-sass.

I can't see any simple fix, but perhaps the !default flags need to be merged back into parsedVars afterwards? The variable names are being tracked alongside, so default flags could be tracked with them too.

EDIT: I've opened this PR with the fix described above 🙂