Add ide-assist: extract_string

[A4-Tacks]

Extract substring inside format string.

Example

fn main() {
    let out = format_args!("foo$0bar$0");
}

->

fn main() {
    let new_str = "bar";
    let out = format_args!("foo{new_str}");
}

[ChayimFriedman2]

Unless you are ready to have wasted work, please don't create PRs for new assists. We already have too many of them and are in the middle of a migration to SyntaxEditor, it's not clear if and when and what we want to accept.

[A4-Tacks]

migration to SyntaxEditor

I created this assist using SyntaxEditor

[ShoyuVanilla]

SyntaxEditor is not finished yet as a foundation for improving our syntax tree manipulation infrastructure. Its current primary goal is to make that work easier by centralizing all mutable syntax tree usage in one place.

Beyond mutable tree handling, we are also planning other changes - such as adopting a Roslyn-style trivia model - which will significantly affect how whitespace and comments are represented and manipulated. The more functionality we build on top of the current infrastructure, the harder this migration will become.