Update syn crate from 1.0.109 to 2.0.101 (#31301)

Nearly all generated by Zed Agent + Claude Opus 4. I just wrote the test
`Args` struct and pointed it at the [2.0 release
notes](https://github.com/dtolnay/syn/releases/tag/2.0.0).

Release Notes:

- N/A

crates/ui_macros/src/dynamic_spacing.rs

@@ -23,7 +23,7 @@ enum DynamicSpacingValue {
 impl Parse for DynamicSpacingInput {
     fn parse(input: ParseStream) -> syn::Result<Self> {
         Ok(DynamicSpacingInput {
-            values: input.parse_terminated(DynamicSpacingValue::parse)?,
+            values: input.parse_terminated(DynamicSpacingValue::parse, Token![,])?,
         })
     }
 }