Add JULIA_VULKAN_LIBNAME for setting a custom libname

[serenity4]

This is a small part of what I did in #34, related to how the libname is picked. Rather than just hardcoding a value for each platform, it allows one to set a custom libname. I also put a more explicit error message, and removed the use of JULIA_VULKAN_SDK_*. So basically, a libname is chosen at compile time, and checked/dlopened at runtime via __init__. Hopefully some of the work of #34 on dynamic library loading will be applicable to 1.6, because it's a pain to change the libname once VulkanCore is compiled (if there are multiple Vulkan implementations you want to test for example, or if you had a bad libname before compiling). I kept a working version of #34 that allows a dynamic loading behavior on >= 1.6 and falls back to what we do now for earlier versions, we can give it a look once 1.6 is out.

[codecov]

Codecov Report

Merging #36 (a348d2e) into master (0bb5151) will decrease coverage by 8.33%.
The diff coverage is 66.66%.

@@             Coverage Diff             @@
##            master      #36      +/-   ##
===========================================
- Coverage   100.00%   91.66%   -8.34%     
===========================================
  Files            2        2              
  Lines           15       12       -3     
===========================================
- Hits            15       11       -4     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
src/LibVulkan.jl	90.90% <66.66%> (-9.10%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0bb5151...a348d2e. Read the comment docs.