WIP: fix(config): mark config.js as a side effect

What does this MR do?

This MR adds config.js to the list of side effects in package.json to avoid that webpack will prune it out during tree-shaking (in gitlab main codebase and anywhere else)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [-] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [-] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [-] If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • [-] GitLab: gitlab!34941 (closed)
  • [-] Customers Portal: mr_url
  • [-] Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.

Note

This change will have no effect on the development environment in Gitlab, BUT it will change the production (and fix some bugs)

like this one: gitlab#222515 (closed) and this one: gitlab#222973 (closed)