Dollars in idents

[som-snytt]

Warns on definitions named with embedded dollars, which should must be written in backquotes, if at all.

Fixes #18234

Test from #18563

[sjrd]

which should be written in backquotes

They should not be written at all, in fact. ;)

[som-snytt]

This proposal aims to accommodate the two views, which are not entirely incompatible, that strict enforcement is required and yet Java doesn't need it so why does Scala?, where the question is posed with a large dosage of skepticism about warnings.

Here, there is no forking compiler option. Definitions with suspect names are disallowed, with a migration warning now and an error later. However, the workaround, to permit the identifier in backquotes, is quite natural and pleasant, as we have all come to savor backquotes in new syntactic contexts. Backquotes are not required for references (usages).

CB c includes the original izumi-reflect example.

A rewrite will be offered in a follow-up. The corner case is pattern variables, where it's not sufficient to simply wrap the identifier in backquotes.