Migrate to dart-sass

[pgoldberg]

Fixes #4032, fixes #4751

Changes proposed in this pull request:

• Migrate from node-sass to dart-sass
• Run sass-migrator to replace / division operators with * or math.div() where appropriate

Reviewers should focus on:

No regressions

Screenshot

[palantirtech]

Thanks for your interest in palantir/blueprint, @pgoldberg! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[adidahiya]

Looks like you made some good progress here. I'm bumping stylelint & stylelint-config-palantir in #5217, so you should get lint passing after that merges.

[adidahiya]

New stylelint config has a bug, see palantir/stylelint-config-palantir#58. I've worked around it with a config override for now.

[adidahiya]

preview builds: docs-app, demo-app

[adidahiya]

There's a bug in menu items that have submenus, probably related to the @extend change. This text shouldn't have an underline:

also, the docs-wide search omnibar isn't highlighting results as expected (the JS functionality still works, but the styles have regressed):

[pgoldberg]

Fixed those in the last commit, although it's not nearly as nice as just extending the compound selectors

Tested with dark mode and intents (comparing against blueprintjs.com) to confirm it still works correctly in those circumstances

docs-app

[adidahiya]

nice, the menu item styles mixin implementation looks good 👍