No outdent at eof

[som-snytt]

Fixes #22332

The reference does not mention <eof>. <outdent> insertion does not require a particular next token, though some next tokens affect it (that is, leading infix or tokens that close an indentation region). It does require a "first token on the next line", for which we may take <eof> as the lack of a token.

Of course, ordinary error messages say eof.

The same text with an opening brace is unchanged:

5 |
  | ^
  | '}' expected, but eof found

[som-snytt]

rebased

[KacperFKorban]

LGTM!