-Xlint:dentation to debug indentation

[som-snytt]

Instead of anticipating many possible ways indentation processing may not match expectation, it would be nice to have a way to lint indentation. -Xlint:dentation. That could be a plug-in that shows an outline view or just -Xprint with indents and outdents highlighted. It could restrict output to regions where warnings/errors were emitted, and just outline previous indents.

Sample where the problematic indentation was not local to the warning/error: https://github.com/lampepfl/dotty/issues/9515

https://github.com/lampepfl/dotty/issues/9822 https://github.com/lampepfl/dotty/issues/9790 https://github.com/lampepfl/dotty/issues/7800

Showing that -Xprint is the go-to tool: https://github.com/lampepfl/dotty/issues/8256

It would be great if messaging by the Lintdentator plugin could incorporate some words from the tickets: Surprising? Perhaps! or Not weird at all! Because Lindt is the chocolate, there might be a better pun on Lindtentation.