Indentation support for prefixed lines

[hyperfekt]

There are various situations in various languages in which lines are prefixed with one or more non-whitespace characters. Often enough, indentation happens not (only) in front of these characters, but also behind them.

It would be great if Visual Studio Code could provide its indentation-supporting features like the Indent/Outdent action, automatic folding ranges, and continued indentation on Enter in these cases.

Examples include block comments in C, which are commonly marked by an asterisk prefix, and bird-style Literal Haskell, where code lines are preceded by a closing angle bracket.

One relevant exploration is the document linked in this comment.

I intend to implement this feature and extend VSCode's extension API to allow language configuration for it.