Twenty Nineteen sass adjustments

[sabernhardt]

Co-authored by @rejaulalomkhan

• Updates package-lock.json after npm audit fix and npm dedupe (which might be done with other npm updates anyway).
• Changes the build to run each command sequentially, so npm run build does not need to be used twice to update the RTL stylesheet.
• Introduces sass:string to prevent deprecation warnings for unquote() global built-in.
• Uses math.div() to prevent deprecation warnings for slash-div calculations.
• Does not replace @import (yet).

Trac 63013

Use of AI Tools

none by me

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[sabernhardt]

Replacing @import is not included in this PR because that is much more involved than the rest.