fix missing vendors, re-vendor with LK4D4/vndr

[thaJeztah]

I didn't see a configuration file for a vendoring tool, so used https://github.com/LK4D4/vndr for now; alternatively the whole vendor directory could be removed

[f2prateek]

We didn't want to vendor dependencies for the cli and examples package, as they're not meant for consumers of the library (e.g. the mold package). Not sure if there's a better way to handle that.

[thaJeztah]

if no specific versions of the dependencies are needed, and people are mainly expected to use this repo as a dependency, perhaps remove the vendor directory altogether?