add `string` as version scheme

[tschmidtb51]

I suggest to add a version scheme string (alternatively it could be named lexicographic or something similar) that is defined as follows:

• Interpret each version as string in UTF-8 representation.
• Compare strings based on characters in lexicographical order. (LC_COLLATE=POSIX)

This equals the result of the Unix sort command.

Flagging @pombredanne for attention

[immqu]

I have created a PR for this issue: aboutcode-org/univers#147