"Fill match arms" assist removes comments

[Veetaha]

We need to preserve the useful trivia already located in the match body, otherwise, the assist totally erases it.

Also, as a side note, it would be better if the match arms body were curly braces because I frequently find myself changing the parentheses to curlies in order to start writing the match arm body.

[matklad]

Impl notes: instead of replacing the whole MatchArmList, we should "append" new arms. This should works similar to ItemList::append_items.

That is, in that edit.rs file we should add

impl ast::MatchArmList {
    #[must_use]
    pub fn append_arms(&self, arms: impl IntoIterator<Item = ast::MatchArm>) -> ast::MatchArmList {

    }
}

[matklad]

(As a general design note, all assists should prefer to copy/move/append existing ranges of syntax nodes, written out by the user, rather than trying to construct fresh nodes)