Add `JS_GetVersion()`

[xeioex]

This is an attempt to make version to be more versatile and more accessible to the C code. I am not sure about specific versioning numbers, I am also fine with the existing scheme with the release date. I can adjust the patch accordingly.

What I need:

1. The simple ability to conditionally compile against a known version (especially taking into account the recent changes) with existing libraries available in the distros. The preferred way for me is to do a simple comparison against a known version.

2. A method to get a string form of the version, to simplify the debugging of user reports.