[request] Implement line-range formatter

[boozook]

Please implement line-range formatter. Currently it's not.

How it (should) works:

In the next file demo.rs with content:

struct Foo {x:i8 ,y  :  i8
       }
struct Bar {x:i8 ,y  :  i8}
    

• select line 6 (with struct Bar {x:i8 ,y : i8})
• ⌘+P, type Format Selection.

[nrc]

This is done on the vscode/rls side, but needs some work in rustfmt - see rust-lang/rls#333 for some discussion.