Support sqlfluff as an alternative formatter

[musjj]

https://github.com/sqlfluff/sqlfluff

In general sqlfluff is just a nicer and more well-mantained formatter that handles edge cases more robustly IMO. Faced a few cases where sql-formatter is just not doing things consistently, for example:

$ echo "CREATE TABLE foobar ( ids UUID[] );" | sql-formatter
CREATE TABLE foobar (ids UUID []);

$ echo "CREATE TABLE foobar ( ids TEXT[] );" | sql-formatter
CREATE TABLE foobar (ids TEXT[]);

In sqlfluff, they're always consistent. The CLI tool is also more CI-friendly as it can print diagnostics if there are any non-conformant sql files:

$ sqlfluff lint test.sql --dialect ansi
== [test.sql] FAIL
L:   1 | P:   1 | LT01 | Expected only single space before 'SELECT' keyword.
                       | Found '  '. [layout.spacing]

sqlfluff can also be configured a lot more flexibly: https://docs.sqlfluff.com/en/stable/reference/rules.html

[senkenn]

I am not having trouble with sql-formatter right now. So I don't plan to support sqlfluff anytime soon. I will consider supporting it as more requests come in.
If we do decide to support more than one formatter, I will allow users to pass format commands, and allow them to use the formatter of their choice.