feat(parser): Properly support semantic versions

[hairyhenderson]

Fixes #42, potentially.

After I realized that the VERSION grammar didn't support anything other than x.y.z form versions, the issues I've been having with prerelease identifiers make much more sense!

I've made a few changes to fix this:

• took the BNF grammar from semver.org and done a direct translation to ANTLR in SemverGrammar.g4
• updated parser/gen.sh to support either antlr4 or antlr commands (it's the latter on my system, but can't assume the same elsewhere)
• added a few new unit tests to confirm correct semver comparison behaviours
• updated dependency to github.com/blang/semver/v4

Hopefully this all makes sense, and can be merged quickly!