Add a builtin:version variable

[krader1961]

The recent discussion in issue #1324 reminded me that I have long thought there should be something like a builtin:version variable that contains the same string emitted by elvish -version. I always worry that the value reported by elvish -version does not match the user's interactive shell. Even if that is unlikely in practice I would be happier asking the user to run echo $version. Also, it would be nice if this information was available in a form useful to an Elvish program so it can test which version is being used; i.e., a pseudo-map that exposes the major, minor, patch levels, etcetera. What I would love is the ability to use the var in a string context that automatically constructs the string from the constituent values of the map while allowing those values to be extracted individually; e.g., $version[major].

[krader1961]

This is now implemented. I forgot to include a "Fixes #1328" line in my commit comment.