Add new DISABLE_SPLIT_LIST_WITH_COMMENT flag.

[jlebar]

DISABLE_SPLIT_LIST_WITH_COMMENT is a new knob that changes the behavior of splitting a list when a comment is present inside the list.

Before, we split a list containing a comment just like we split a list containing a trailing comma: Each element goes on its own line (unless DISABLE_ENDING_COMMA_HEURISTIC is true).

Now, if DISABLE_SPLIT_LIST_WITH_COMMENT is true, we do not split every element of the list onto a new line just because there's a comment somewhere in the list.

This mirrors the behavior of clang-format, and is useful for e.g. forming "logical groups" of elements in a list.

Note: Upgrading will result in a behavioral change if you have DISABLE_ENDING_COMMA_HEURISTIC in your config. Before this version, this flag caused us not to split lists with a trailing comma and lists that contain comments. Now, if you set only that flag, we will split lists that contain comments. Set the new DISABLE_SPLIT_LIST_WITH_COMMENT flag to true to preserve the old behavior.

[jlebar]

cc @ptillet

[jlebar]

Thank you for the review!

[jlebar]

Woo, thank you for merging!

[jlebar]

Note to self: This branch is referred to in the Triton pre-commit, so I should not delete it. :)