fmt: keep {} in string interpolation for chained CallExprs (#8422)

vlib/v/ast/str.v

@@ -158,6 +158,9 @@ pub fn (lit &StringInterLiteral) get_fspec_braces(i int) (string, bool) {
 				CallExpr {
 					if sub_expr.args.len != 0 {
 						needs_braces = true
+					} else if sub_expr.left is CallExpr {
+						sub_expr = sub_expr.left
+						continue
 					}
 					break
 				}

vlib/v/fmt/tests/string_interpolation_literal_keep.vv

@@ -9,5 +9,5 @@ fn main() {
 	d := 'a: ${a:5s} b: ${b:-5s} i: ${i:20d}'
 	println('a: $a $b xxx')
 	eprintln('e: $e')
-	println(d)
+	_ = ' ${foo.method(bar).str()} '
 }