[issue_934] from_shebang tera function uses starts_with

[adam-tokarski]

Fixes #934

This PR allows to cover versioning of language executables, like it can happen for Python. Currently there is python, python2 and python3 in this language env. Could just add python2.7, but we should add new env item for each new Python version, like python3.4, python3.10, etc. This soon may be somewhat cumbersome, so there is a proposition just to match using starts_with which should cover such all things. Main drawback is that it could cover things like pythonxxx, but not sure if that's a very big problem.

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It's merely my second touch of rust at all (not mentioning the tera), so if there would be some better way to handle that, I'd love to hear that.