Function extraction_target 

fn extraction_target(
    node: &SyntaxNode,
    selection_range: TextRange,
) -> Option<FunctionBody>

Try to guess what user wants to extract

We have basically have two cases:

• We want whole node, like loop {}, 2 + 2, { let n = 1; } exprs. Then we can use ast::Expr
• We want a few statements for a block. E.g.

  fn foo() -> i32 {
    let m = 1;
    $0
    let n = 2;
    let k = 3;
    k + n
    $0
  }