[core] fix CSS syntax in box-shadow variables

[adidahiya]

Fixes #5987

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

• Refactor generate-css-variables implementation into testable functions
• Add jest + ts-jest testing infra to node-build-scripts package
• Refactor generate-css-variables implementation to use postcss-simple-vars instead of get-sass-vars
  • The latter proved difficult to test via Jest (it wouldn't import successfully either as CommonJS or ESM). get-sass-vars also has little usage and probably is a little too complex / magical for us.
  • This migration required implementing a bit of the math logic which get-sass-vars was doing for us, though. I've added some special case handling for function calls in variable initializers of the form rgba($black, 0.1) where the first argument is a hex color value -- we now replace those variables with literal r, g, and b values like rgba(17, 20, 24, 0.1).
• Add regression tests for Regression in box-shadow Sass variable syntax #5987

Reviewers should focus on:

Regression tests cover the functionality that we care about.

I compared the Sass variables output against this last known working version: https://unpkg.com/browse/@blueprintjs/core@4.16.3/lib/scss/variables.scss

Screenshot

N/A

[adidahiya]

This commit contains the meaningful changes in this PR: ab10086

[adidahiya]

remove dependency on @blueprintjs/colors

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[adidahiya]

Revert "bump CI cache key"

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[adidahiya]

fix getRootDir()

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