feat: pkg-config

[G4Vi]

Adds pkg-config configs for dynamic and static builds of libextism. Example usage is shown in libextism/Makefile.

I added the .skel versions of the .pc files to non-MSVC releases. Ideally, the extism cli when installing the lib would set the PREFIX and install them.

[zshipko]

Works great, it's a major improvement to be able to locate the installation!

Typically I have seen these files with the .pc.in extension but since they're renamed before getting copied it doesn't really matter either way.

[G4Vi]

Typically I have seen these files with the .pc.in extension but since they're renamed before getting copied it doesn't really matter either way.

I could not think of the convention in the moment. Fixed!