Support relative base path on dev server

[juan-rvty]

Description

Make

 const config = {
  base: './'
}

supported in the dev server.

Use case:

Our app always runs inside iframes, even during development.
This relates to:
#14105
but in my case the uri is not known before hand.
Also relates to:
#11142

Suggested solution

Allow relative base path in dev mode.

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.