Support file URLs when configuring multiple transports

[haines]

pino currently supports file URLs in single transports

pino.transport({ target: "file:///path/to/transport.js" });

but in multiple transports it does not:

pino.transport({ targets: [{ target: "file:///path/to/transport.js" }] });

throws

{
  code: "ERR_INVALID_URL",
  input: "file://file:///path/to/transport.js",
}

This PR adds a check for the presence of the file:// prefix before prepending it to the target.

[mcollina]

lgtm