util: ignore invalid format specifiers

In util.format, if a percent sign without a known type is encountered, just print it instead of silently ignoring it and the next character.

Fixes: https://github.com/nodejs/node/issues/13665

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

util