Support reading the version from a using directive

[lefou]

the directive will looks like this:

//> using mill.version 0.11.8

IMHO, it would be enough to search for it in the first 10 lines (head).

Also supporting a mill.minVersion would be nice, so we could use whatever version (preferrably the latest) that matches.

[lefou]

• Related to Support an alternative to $-imports com-lihaoyi/mill#3062