fix(ts): prevent ts-node to register twice

[kevinmarrec]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

This add a guard to prevent ts-node registration to happen more than once.
We had a similar guard few Nuxt versions ago but it seems to have been unintentionally removed with last refactors.

Fixes https://github.com/nuxt/nuxt.js/issues/5626

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
• All new and existing tests are passing.