Ensures that files either use tabs for indentation or spaces for indentation,
but don't switch between the two within the same file.
This doesn't fix the whitespace, it's a simple check to flag it up
on the assumption that it's better to use an editor or code formatter
to clean up the file.
Elsewhere in the CI we use the luacheck tool - while that can detect
mixing tabs and spaces in a single line's indent, it doesn't check for
inconsistent indentation within a file.
