Feature Request: Please add .commands support for DuckDB dialect

[pavlokurochka]

Describe the Feature
Currently .commands break formatting of DuckDB SQL files

Why do you want this feature?
It'll be much more convenient not to have to comment them out to be able to format.

[nene]

This is currently in similar vein to how \commands in PostgreSQL are not supported. And I guess there are other dialects with similar special command-line commands.

Looks like this page documents the DuckDB syntax for these commands.

Because DuckDB commands begin with a . it's particularly tricky to support because . is already a special symbol in SQL. Even . at the beginning of a line is perfectly valid SQL. So I need to somehow understand when dot at the beginning of a line is inside an SQL expression, like:

SELECT tbl
.help
FROM tbl;

Versus when it's not inside an SQL expression:

SELECT tbl;
.help

Ideally I'd like to distinguish this at the Lexer level, but because of the above, that's not possible. So I would need to do this inside the parser, but until now the parser has been completely dialect-agnostic. I'm not really keen to add such a conditional logic (if DuckDB, parse it differently) in there.

Hmm... possibly I can do a separate pass over the tokens (like I already do for some dialects) to distinguish between the different uses of . before passing it to parser.

In any case, solving the \command problem in PostgreSQL is currently with a higher priority.