yeast-macros: desugar 'field: @cap' to 'field: _ @cap'

When a field pattern has a bare capture with no preceding pattern
atom (i.e. `foo: @bar`), implicitly use a true wildcard (`_`,
match_unnamed: true) as the node pattern, making it equivalent to
`foo: _ @bar`.

This is a convenience shorthand: in practice every `field: _ @cap`
in the Swift rules can now be written more concisely as `field: @cap`.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

shared/yeast-macros/src/parse.rs

@@ -141,7 +141,12 @@ fn parse_query_fields(tokens: &mut Tokens) -> Result<Vec<TokenStream>> {
             // Parse the field's pattern. To support repetition like
             // `field: (kind)* @cap`, parse the atom first, then check for
             // a quantifier, and lastly handle a trailing `@capture`.
-            let atom = parse_query_atom(tokens)?;
+            // `field: @cap` is sugar for `field: _ @cap`.
+            let atom = if peek_is_at(tokens) {
+                quote! { yeast::query::QueryNode::Any { match_unnamed: true } }
+            } else {
+                parse_query_atom(tokens)?
+            };
             if peek_is_repetition(tokens) {
                 let rep = expect_repetition(tokens)?;
                 let elem = quote! {