feat: add support for sqlfluff

[nettoyoussef]

This pull request adds support for SQLFluff.
There are some problems though. The only way I found to format files with a custom configuration template (which is probably what most users working on production databases would like) was:

(add-hook 'sql-mode-hook
  (lambda ()
    (define-format-all-formatter sqlfluff
      (:executable "sqlfluff")
      (:install "pip install sqlfluff")
      (:languages "SQL")
      (:features)
      (:format (format-all--buffer-easy executable 
                 "fix"
                 "--config=/path/to/config.ctg"
                 "--nocolor" "-")
      )
    )
  )
)

[lassik]

Thank you. Looks good to me.

The only thing is that --dialect=postgres and --config=/path/to/config.ctg should not go into the default options passed by format-all. Users can add their own command line arguments like this:

(add-hook 'sql-mode-hook
          (lambda ()
            (setq format-all-formatters
                  '(("SQL" (sqlfluff "--dialect=postgres" "--config=/path/to/config.ctg"))))))

[nettoyoussef]

I am glad to contribute, I've been a user for several years by now :)

The code above doesn't seem to work for me. I tried running format-all-buffer with (setq format-all-debug t) and:

(add-hook 'sql-mode-hook
  (lambda ()
    (setq format-all-formatters
      '(("SQL" (sqlfluff "fix" "--config=/path/to/config.ctg" "--ignore=parsing,templating" "--nocolor" "-")))
    )
  )
)

Which gave the output:

Format-All: Formatting query_temp.sql as SQL using sqlfluff fix --config\=/path/to/config.ctg --ignore\=parsing\,templating --nocolor -

condition-case: Symbol’s function definition is void: nil

Also, is there a way to use the linter output even when there is an error?
SQLFluff has a kind of output called unfixable, which is just the linter saying that it could not implement all rules, but it still provides better output than before.
I tried a hack with the code below, redirecting the stderr to /dev/null, but it didn't work:

(add-hook 'sql-mode-hook
  (lambda ()
    (define-format-all-formatter sqlfluff
      (:executable "sqlfluff")
      (:install "pip install sqlfluff")
      (:languages "SQL")
      (:features)
      (:format (format-all--buffer-easy executable 
                 "fix"
                 "--config=/path/to/config.ctg"
                 "--ignore=parsing,templating"
                 "--nocolor" 
                 "-"
                 "2>/dev/null" ;; avoid error checking, a bit risky
                )
      )
)))

Regarding --dialect=postgres, while I agree that the implementation should not be opinionated, the linter doesn't run without a dialect, so if we don't include it we are basically forcing users to implement a correction/configuration.