Accept and ignore a shebang (`#!`) before a YAML frontmatter (`//|`)

[lefou]

Java since version 11 allows a shebang in source files. Also Kotlin scripts (.kts) accept it. To allow Java/Kotlin scripts to also being used as Mill single file scripts Mill needs to accept (and ignore) a shebang in the first line, but still read the YAML frontmatter starting in the second line.

A second use case would be self-executable single file scripts starting with

#!/usr/bin/env mill