feat: support custom command to move item up/down

[rchl]

Implement handling for custom request to move item up and down.

Tested with simple case like:

fn two() {
    println!("Hello, world1!");
}

fn main() {
    println!("Hello, world!");
}

and moving fns around.

Note that rust-analyzer returns custom "snippet" text edits. That means that one or more of the edits (in this case one) will include a snippet like $0 or ${0:foo}. I'm implementing opt-in support for that in LSP which this change depends on.

The point of the snippet is that the cursor position is moved together with moved function/item which otherwise would just stay in the old place.

Based on rust-analyzer extension code:

• move commands: https://github.com/rust-lang/rust-analyzer/blob/0cd68bfed3e31c034a5ead664ac8ad84d8bc06fd/editors/code/src/commands.ts#L163-L188

Fixes #28