ClickHouse: # comments are not recognized #2673

ClickHouse accepts # followed by a space as a single-line comment
(https://clickhouse.com/docs/sql-reference/syntax#comments), but the
ClickHouse dialect tokenizer rejects it:

  SELECT 1 # it's a comment
  -- Unterminated string literal at Line: 1, Column: 14

The tokenizer only treats # as a comment start for Snowflake, BigQuery,
MySQL and Hive. Found by the lexer-comparison fuzz target from #2646.