Module Params' proposal import syntax

I previously implemented the syntax in "Proposal: Module and Package Changes" [1]:

```
import [map, map2] from JsonDecode as JD
```

However, we decided [2] to use the one that appears in "Proposal: Module Params" [3]:

```
import JsonDecode as JD exposing [map, map2]
```

The new implementation also now supports comments and newlines between all the tokens.

[1] https://docs.google.com/document/d/1E_77fO-44BtoBtXoVeWyGh1xN2KRTWTu8q6i25RNNx0/edit
[2] 405410612
[3] https://docs.google.com/document/d/110MwQi7Dpo1Y69ECFXyyvDWzF4OYv1BLojIm08qDTvg/edit

crates/compiler/fmt/src/spaces.rs

@@ -572,7 +572,11 @@ impl<'a> RemoveSpaces<'a> for ValueDef<'a> {
             } => ModuleImport {
                 name: name.remove_spaces(arena),
                 alias: alias.map(|alias_name| alias_name.remove_spaces(arena)),
-                exposed: exposed.remove_spaces(arena),
+                exposed: if let Some((spaces, exposed)) = exposed {
+                    Some((spaces, exposed.remove_spaces(arena)))
+                } else {
+                    None
+                },
             },
         }
     }