Refactor support for converting enum variants with fields to structs

Use case: Rustdoc has a struct with a bunch of repeated fields:

pub enum DocFragment {
    SugaredDoc(usize, Span, String),
    RawDoc(usize, Span, String),
    Include(usize, Span, String, String),
}

I want to turn that into

pub struct DocFragment {
    line_number: usize,
    span: Span,
    doc: String,
    kind: DocKind,
}

pub enum DocKind {
  SugaredDoc,
  RawDoc,
  Include { filename: String },
}

and update all usages.

Probably this can't be 100% automated - the field names in particular need user input and refactoring out the enum might be tricky. But it would be nice to have some amount of support.

The issue provides the Rustdoc DocFragment enum example and the desired struct-plus-DocKind shape, but names no rust-analyzer files, tests, or entry points. Start by locating the refactoring support for enum variants with fields; done means providing useful conversion support and updating affected usages, with user-supplied field names where automation cannot decide.