src: use String::NewFromUtf8 in ProcessEmitWarningGeneric

[codebytere]

Description of Change

Previously, the new warning string in ProcessEmitWarningGeneric was allocated using String::NewFromUtf8, but type and warning, despite also being const char*, were instead allocated using String::NewFromOneByte. I couldn't find a semantic reason for using differing string allocation functions, so this PR improves consistency by using String::NewFromUtf8 for all three.

Checklist

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

[nodejs-github-bot]

Sadly, an error occurred when I tried to trigger a build. :(
CI: https://ci.nodejs.org/job/node-test-pull-request/23853/
CI: https://ci.nodejs.org/job/node-test-pull-request/23854/
CI: https://ci.nodejs.org/job/node-test-pull-request/23859/

[bnoordhuis]

It's because while the warning is an arbitrary string (in theory - in practice there are just a handful), type and code are fixed ASCII strings.

String::NewFromOneByte() is a little faster than String::NewFromUtf8() because it doesn't attempt to decode UTF-8.