fix regression that broke string.format with list objects

String formatting should validly assume that printing a list means
printing the list itself. Instead, something like this broke:

'one is: @0@ and two is: @1@'.format(['foo',  'bar'], ['baz'])

which would evaluate as:

'one is: foo and two is: bar'

or:

'the value of array option foobar is: @0@'.format(get_option('foobar'))

which should evaluate with '-Dfoobar=[]' as

'the value of array option foobar is: []'

But instead produced:

meson.build:7:0: ERROR: Format placeholder @0@ out of range.

Fixes mesonbuild#9530

mesonbuild/interpreter/primitives/string.py

@@ -13,6 +13,7 @@
     MesonOperator,
     FeatureNew,
     typed_operator,
+    noArgsFlattening,
     noKwargs,
     noPosargs,
     typed_pos_args,
@@ -85,6 +86,7 @@ def startswith_method(self, args: T.Tuple[str], kwargs: TYPE_kwargs) -> bool:
     def endswith_method(self, args: T.Tuple[str], kwargs: TYPE_kwargs) -> bool:
         return self.held_object.endswith(args[0])
 
+    @noArgsFlattening
     @noKwargs
     @typed_pos_args('str.format', varargs=object)
     def format_method(self, args: T.Tuple[T.List[object]], kwargs: TYPE_kwargs) -> str: