Implement --fix for avoid_single_cascade_in_expression_statements lint

[cpelling]

It would be useful for dartfmt --fix to automatically fix violations of the lint avoid_single_cascade_in_expression_statements.

[matanlurey]

It is extremely unlikely dartfmt would have knowledge of or how to fix lints (separate ecosystem).

[cpelling]

For context, there's a discussion about enforcing this lint internally. Having an option similar to --fix-optional-new for automatically fixing cases which would violate that lint seems like it would be useful. I'm willing to help with the work.

[munificent]

Yeah, this particular lint is one that's easy to recognize syntactically, which is why I told @cpelling that it was a good candidate for a --fix.

[munificent]

Closing since we're getting rid of dart format --fix in favor of dart fix: #1153