Expose Parser::into_comments

[xitep]

• Back then we didn't make the method public to guarantee the binary search over the comments
• Later in the process we skipped adding out-of-order comments: https://github.com/apache/datafusion-sqlparser-rs/blob/main/src/ast/comments.rs#L40
• therefore exposing the method should be just fine now
• exposing the method allows to use the parser (e.g. extract the statements individually) and still obtain the Comments instances without calling parse_sql_with_comments

[iffyio]

LGTM! Thanks @xitep!