fix: include src dir in npm package

What does this MR do?

In !1426 (diffs), we tried to exclude specs, stories and snapshots from the npm package but it looks like the changes we made are actually preventing the whole src/ directory from being installed along with the @gitlab/ui package: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/jobs/564488721

This MR reverts those changes to ensure that src/ is properly included in the package.

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: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• [-] Added the ~"component:*" label(s) if applicable.