feat: add .bats file type to shell

[SethFalco]

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Adds the .bats file type and maps it to shell.

BATS (Bash Automated Testing System) is a notable project with > 5k stars on GitHub. ⭐ Given the type of file it is, of the existing icons, shell is the most appropriate.

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They do technically have their own icon, so it would be even better if .bats was specifically bound to the bats icon, however not sure if it's worth including the icon in this theme. Thoughts welcome!

• https://github.com/bats-core/bats-core/blob/master/docs/source/assets/light_mode_bat.svg
• https://github.com/bats-core/bats-core/blob/master/docs/source/assets/dark_mode_bat.svg