Warn for unindented case switches

Alternatively, shfmt recommends maintaining the same indentation level for each pattern as the opening "case" keyword. Yes, this is a somewhat subjective choice, but as there is now a semistandard tool for styling shell script code, we might as well follow it:

https://github.com/mvdan/sh

So bashate could offer an optional setting for a case indentation preference, but should default to shfmt style so that these two powerful tools do not clash.

shfmt is a new one, hadn't heard of it. I don't know how widespread their standard is.

OTOH, Google's style guide (which a lot of individuals and projects follow), recommends indenting one level:

""""""
    Indent alternatives by 2 spaces.
    A one-line alternative needs a space after the close parenthesis of the pattern and before the ;;.
    Long or multi-command alternatives should be split over multiple lines with the pattern, actions, and ;; on separate lines.

The matching expressions are indented one level from the 'case' and 'esac'. Multiline actions are indented another level. In general, there is no need to quote match expressions. Pattern expressions should not be preceded by an open parenthesis. Avoid the ;& and ;;& notations.
""""""

https://google.github.io/styleguide/shell.xml#Case_statement