signal: Add return type annotation for savgol_coeffs

In the file `_savitzky_golay.pyi`.

Additionally add `Literal` type annotation for the `use` parameter which
can only be "conv" or "dot".

scipy-stubs/signal/_savitzky_golay.pyi

@@ -1,13 +1,19 @@
+from typing import Literal, TypeAlias
+
+import numpy as np
+import numpy.typing as npt
 from scipy._typing import Untyped, UntypedArray
 
+_Array_fc_1d: TypeAlias = np.ndarray[tuple[int], np.dtype[np.inexact[npt.NBitBase]]]
+
 def savgol_coeffs(
     window_length: int,
     polyorder: int,
     deriv: int = 0,
     delta: float = 1.0,
     pos: int | None = None,
-    use: str = "conv",
-) -> UntypedArray: ...
+    use: Literal["conv", "dot"] = "conv",
+) -> _Array_fc_1d: ...
 def savgol_filter(
     x: Untyped,
     window_length: int,