Fix shell scripts missing tags

[Enzime]

This PR changes it so that tags_from_path returns the tag from the shebang if the file is a shell script.

example.sh:

#!/bin/bash

tags_from_path("example")
Without this PR

{'file', 'text', 'executable', 'shell'}

With this PR

{'file', 'text', 'executable', 'shell', 'bash'}

[asottile]

this isn't correct

it should have been obvious from the tests and comments that what you're doing is wrong