ClickHouse supports PostgreSQL-style heredoc syntax: $tag$ string

ClickHouse supports PostgreSQL-style heredoc syntax: $tag$ string contents... $tag$, also known as dollar-quoted string literals. In previous versions, there were fewer restrictions on tags: they could contain arbitrary characters, including punctuation and whitespace. This introduces parsing ambiguity with identifiers that can also start with a dollar character. At the same time, PostgreSQL only allows word characters for tags. To resolve the problem, we now restrict heredoc tags only to contain word characters. Closes #84731. #84846 (Alexey Milovidov).