Updates node-sass to 4.12 and regenerates lockfiles

[aledileo]

Solves: #961
node-sass 4.12 works fine with node 12, the issue originated from having both yarn and npm lockfiles and them using different versions of the library. May I suggest sticking to only one package manager? It would decrease the amount of bugs like this one

[coveralls]

Pull Request Test Coverage Report for Build 1058

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 53.56%

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Totals
Change from base Build 1056:	0.0%
Covered Lines:	2643
Relevant Lines:	4887

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💛 - Coveralls

[klemmchr]

I would also favor for using just one package manager.

[Bjego]

Instead of updating node-sass - I would port to dart sass. Node-Sass is a legacy implementation and has been replaced by dart-sass.

Dart Sass is the primary implementation of Sass, which means it gets new features before any other implementation. It's fast, easy to install, and it compiles to pure JavaScript which makes it easy to integrate into modern web development workflows. Find out more or help out with its development on GitHub.

https://sass-lang.com/dart-sass

And node-sass is based on the C-Based library "Lib-Sass"

Node-sass is a library that provides binding for Node.js to LibSass, the C version of the popular stylesheet preprocessor, Sass.
https://github.com/sass/node-sass

I've created an issue for that #986

[knsv]

There can be only one. Let's use yarn and remove package-lock.json