bug#55991: 29.0.50; Wish list: detect and ask user if to change indentation or settings

[Jean Louis <bugs@gnu.support>]

For clarification:

"old source files" -- are those made in older times, most often with
different editors, different indentation styles, in different
programming languages.

"new style" -- it is relevant only to such old source files, as Emacs
will automatically impose indentation

However, old source files shall sometimes remain how they are when
opened. There may cultural and habitual reason for the files to remain
not indented.

When user opens such old file, various modes in Emacs are going to
start enforcing the indentation on user.

Users should be warned if they should allow Emacs to change even those
invisible characters or if they should adopt the new indentation or
not when editing such files.

For example, instead of TABS, an Emacs mode may start enforcing
spaces without asking user. Other editor notepadqq asks user if that
change should take place.

1) It is possible to make function to recognize that file had tabs
   instead of spaces, and user should be asked about it.

2) It is possible to recognize that file was not indented by the
   mode's standard, and to ask user if new indentation should be
   adopted.

Jean

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