Changing screenshot algorithm from md5 to sha256 will allow compiler to run in FIPS-180-3 Compliant build containers

Change request: 

Can we make a change here: 

https://github.com/stenciljs/core/blob/6e587615c11b835ed4677fb7162590a02cb35c9b/src/screenshot/screenshot-compare.ts#L25

from md5 to sha256 as md5 is not available in environments that need FIPS compliance (or have restrictions against older hash functions with collision risks) 

I'm happy to raise a PR for this change if accepted 