handle skip_macro_invocations from config file

[calebcartwright]

fixes #5816

We need to keep the FromStr impl for MacroSelector (e.g. to support command line config args), so seems reasonable to just have the serde deserializer use that impl too.

I'm not sure whether there's a better/simpler approach to achieve that (e.g. an item-level serde attribute), but adding our own Deserialize impl seems trivial enough

[ytmimi]

Might be tricky to write a system test for this since skip_macro_invocations is unstable, and stable rustfmt won't actually set the value it finds in the rustfmt.toml.

Mismatch at tests/source\skip_macro_invocations\config_file.rs:1:
 // rustfmt-config: issue-5816.toml
 
 fn main() {
-    println!(             "Hello, world!");
+    println!("Hello, world!");
     let x = 7;
 }

[calebcartwright]

Nuts. Will circle back to this later when I can

[ytmimi]

I'm not sure I knew about // rustfmt-unstable: true, so I learned something! Changes look good to me!