New assist - inline if with outer else

[FMorschel]

Could we add the option to join inner if with else:

if (a) {
} else {
  if (b) {
  } else if (c) {
  }
}

Could become:

if (a) {
} else if (b) {
} else if (c) {
}

Originally posted by @FMorschel in #56715 (comment)

I made a CL https://dart-review.googlesource.com/c/sdk/+/391021

[dart-github-bot]

Summary: The user proposes adding an assist to the Dart editor that automatically combines nested if statements with an outer else into a single chain of else if statements. This would simplify code structure and improve readability.

[scheglov]

Look useful to me!