fmt: keep braces around @ escaped fields in string interpolation (#8488)

vlib/v/ast/str.v

@@ -165,6 +165,10 @@ pub fn (lit &StringInterLiteral) get_fspec_braces(i int) (string, bool) {
 					break
 				}
 				SelectorExpr {
+					if sub_expr.field_name[0] == `@` {
+						needs_braces = true
+						break
+					}
 					sub_expr = sub_expr.expr
 					continue
 				}

vlib/v/fmt/tests/string_interpolation_literal_keep.vv

@@ -10,4 +10,5 @@ fn main() {
 	println('a: $a $b xxx')
 	eprintln('e: $e')
 	_ = ' ${foo.method(bar).str()} '
+	println('(${some_struct.@type}, $some_struct.y)')
 }