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
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
util