only enable linenoise for -d:nimUseLinenoise

[timotheecour]

• refs: 2b368bc#commitcomment-37273026
• supersedes [superseded] niminst: make linenoise.h optional #13413
• comments out copySrc(libpath / "lib/wrappers/linenoise/linenoise.h") from niminst since this was causing issues
• users who need linenoise support now can't do it from a nim release (sadly...) but they can do it from a locally built nim by passing -d:nimUseLinenoise when building nim, eg via either:
• ./koch boot -d:release -d:nimUseLinenoise
• nim c -o:bin/nim_temp -d:release -d:nimUseLinenoise --lib:lib compiler/nim.nim

I've renamed -d:useLinenoise (which wasn't used since a few commits ago) to -d:nimUseLinenoise to adhere to namespace prefix rule but -d:useLinenoise will continue to work for backward compatibility

future PR's could consider something like: -d:useLinenoise:path/to/linenoise_header so that it could work for a nim release too, pointing to an external path