Fix insertion of using in applications with trailing lambda syntax

[mbovel]

Fixes #22731.

Work-in-progress.

Started during today's Compiler Spree with @ajafri2001.

[som-snytt]

I'm dubious about adding "mechanism" to track the user syntax. For rewriting "unused imports", I spent 100 LOC to inspect the text, since an import is not precisely represented in the trees. My thinking was that, as with error messages, the cost should be deferred (and kept off the happy path). It sounds easier to look for paren or brace (but I didn't try it).

OTOH, when I needed ApplyKind for something, it was so clever and handy. Maybe this will pay for itself, too.

[mbovel]

Re-inspecting text sounds harder to maintain as it duplicates knowledge and assumptions about the syntax that need to be kept in sync with the parser.

On the other hand, I hear you about the importance of keeping the happy path pristine.

I don't know; what do you think @hamzaremmal and @odersky? Would that be okay to always save information about application syntax, or would that be better to re-derive this information from source when needed?