Decouple from Julia version

[davidanthoff]

Medium term we would like to static compile the language server. That will require that CSTParser.jl's behavior does not depend on the Julia version it is running on. The statically compiled LS will for example be compiled with Julia 1.4.0, but it needs to be able to parse source code with the rules of say Julia 1.0.0 if asked to do so.

I think the concrete steps here are two:

• Remove more or less all VERSION checks in the code
• Add a parameter to the main entry point functions that specifies which Julia version syntax CSTParser.jl should use for the parsing. The parameter could default to the version of Julia that is currently running, I guess?

The latter would then for example lead to a situation that if I run CSTParser.jl on Julia 1.4, but parse a file and tell it to use Julia 1.0 syntax, it would identify syntax elements as errors that were introduced to Julia post 1.0.