chore: Switch storybook/webpack to dart sass
What does this MR do?
chore: Switch storybook/webpack to dart sass
First step towards a node-sass-free world
chore: Remove node-sass-magic-importer
Has not been in use since 46b1b58c, so we can just remove it.
chore: Replace markdown-loader-jest
I don't see any tests that would rely on loading rendered markdown
Does this MR meet the acceptance criteria?
Conformity
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Code review guidelines. 
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GitLab UI's contributing guidlines. 
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If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer. 
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If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer. 
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If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/uipackage can be upgraded quickly after the changes are released:- 
GitLab: mr_url 
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CustomersDot: mr_url 
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Status Page: mr_url 
 
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Added the ~"component:*"label(s) if applicable.
Security
If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:
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Label as security and @ mention @gitlab-com/gl-security/appsec
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Security reports checked/validated by a reviewer from the AppSec team 
Accessibility
If this MR adds or modifies a component, take a few moments to review the following:
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All actions and functionality can be done with a keyboard. 
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Links, buttons, and controls have a visible focus state. 
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All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-labelfor icons that have meaning or perform actions.
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Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false"toaria-expanded="true"when an accordion is expanded.
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Color combinations have sufficient contrast. 
Edited  by Lukas Eipert