Add support for ignoring DDL statements using -- sqlc:ignore.

[sgielen]

NOTE: This PR is in a draft state, because it still requires (at the very least) documentation updates.

This PR adds support for commenting out parts of the schema. This is most useful in the case where the schema is defined by migrations, and such migrations contain queries that are not yet supported by sqlc. See for example #3129, #1756.

By wrapping such unsupported queries in sqlc:ignore comments, they are ignored and the rest of the schema can be parsed appropriately.

Example usage in a migration file:

CREATE TABLE person_email (
  email TEXT NOT NULL
);

-- sqlc:ignore until https://github.com/sqlc-dev/sqlc/issues/3129 is resolved
INSERT INTO person_email (`email`)
  SELECT pe.email
  FROM person AS p
  JOIN JSON_TABLE(p.emails, '$[*]' COLUMNS(email TEXT PATH '$')) AS pe;
-- sqlc:ignore end

ALTER TABLE person_email ADD PRIMARY KEY (`email`);

See #3129 for a full use-case.

Since the lines are actually cleared, instead of removed, errors on queries below the sqlc:ignore end marker are still shown with the correct line number.

[sgielen]

@kyleconroy if you agree with this addition, then I would propose to add some documentation regarding this in https://docs.sqlc.dev/en/stable/howto/ddl.html. I can do that too within this PR, but wanted to await your thoughts first.

[sgielen]

@kyleconroy what do you think about this addition?

[sgielen]

@kyleconroy another bump - marking as non-draft in case that helps