Support extract function

[kulikg]

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Feature description

Hi,

I would really love to see this one working in rustaceanvim: https://rust-analyzer.github.io/book/assists.html#extract_function

I took a quick look into it. As you can see the extract function will quit right away if the selection range is empty:
https://github.com/rust-lang/rust-analyzer/blob/master/crates/ide-assists/src/handlers/extract_function.rs#L67

While rustaceanvim is not sending the selection range to the language server:
https://github.com/mrcjkb/rustaceanvim/blob/master/lua/rustaceanvim/commands/code_action_group.lua#L400

Actually I've patched code_action_group() quickly, and it seems the rest is fine, the lsp response is handled ok.

[mrcjkb]

Opened an issue for this: #800