Add syntax highlight to all shell code blocks

[Avasam]

• Using bash by default, powershell for Windows-specific
• Added missing start-of-line markers

Please read #9434 (comment) as for the motivation why, as well as why use bash specifically (mainly for semantic reasons)

As for using powershell for Windows-specific commands over cmd/batch:

• powershell is meant to replace Command Prompt
• is the default in Microsoft's own IDEs

Full explanation with examples (linked comment)

As far as https://github.com/github/linguist/blob/master/lib/linguist/languages.yml#L6204 is concerned, shell, sh, shell-script, bash and zsh are all the same, and look fine.

bash

(.venv3)$ python tests/get_external_dependencies.py <third_party_stub>  # List external dependencies for <third_party_stub>
(.venv3)$ python tests/get_external_dependencies.py <third_party_stub1> <third_party_stub2>  # List external dependencies for <third_party_stub1> and <third_party_stub2>
(.venv3)$ python tests/get_external_dependencies.py  # List external dependencies for all non stubs packages in typeshed
(.venv3)$ pip install $(python tests/get_external_dependencies.py <third_party_stub>)  # Install non-types dependencies

shell

(.venv3)$ python tests/get_external_dependencies.py <third_party_stub>  # List external dependencies for <third_party_stub>
(.venv3)$ python tests/get_external_dependencies.py <third_party_stub1> <third_party_stub2>  # List external dependencies for <third_party_stub1> and <third_party_stub2>
(.venv3)$ python tests/get_external_dependencies.py  # List external dependencies for all non stubs packages in typeshed
(.venv3)$ pip install $(python tests/get_external_dependencies.py <third_party_stub>)  # Install non-types dependencies

powershell

(.venv3)$ python tests/get_external_dependencies.py <third_party_stub>  # List external dependencies for <third_party_stub>
(.venv3)$ python tests/get_external_dependencies.py <third_party_stub1> <third_party_stub2>  # List external dependencies for <third_party_stub1> and <third_party_stub2>
(.venv3)$ python tests/get_external_dependencies.py  # List external dependencies for all non stubs packages in typeshed
(.venv3)$ pip install $(python tests/get_external_dependencies.py <third_party_stub>)  # Install non-types dependencies

empty

(.venv3)$ python tests/get_external_dependencies.py <third_party_stub>  # List external dependencies for <third_party_stub>
(.venv3)$ python tests/get_external_dependencies.py <third_party_stub1> <third_party_stub2>  # List external dependencies for <third_party_stub1> and <third_party_stub2>
(.venv3)$ python tests/get_external_dependencies.py  # List external dependencies for all non stubs packages in typeshed
(.venv3)$ pip install $(python tests/get_external_dependencies.py <third_party_stub>)  # Install non-types dependencies

It doesn't outright break the syntax, if anything it highlights that you need to replace this part. It also correctly handles comments and inline commands.

Editors may have different implementations. This is in VSCode with popular markdown syntax highlight (and github-like previewer) that I use:

This is with plugins disabled:

You can see here that "shell" isn't naturally supported in the preview. (code syntax highlight has no difference). But I wouldn't use that as an argument.
Of course the exact look will depend on the editor's implementation.
In any case, having a language specifier here only seems better than no language specifier.

Another reason to use bash specifically, from a purely semantic point of view, is that the doc seems to assume all commands are to be run on linux, where afaik bash (or bash-like) is most often the default. (and bash-like code usually works in powershell on Windows)

[AlexWaygood]

Thanks!