http2: remove regular-file-only restriction

Requiring `respondWithFile()` to only work with regular files
is an artificial restriction on Node’s side and has become unnecessary.

Offsets or lengths cannot be specified for those files,
but that is an inherent property of other file types.

PR-URL: https://github.com/nodejs/node/pull/18936


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>