[typescript] Allow module augmentation for Mixins

[mnajdova]

I noticed while providing a solution to #32721 that we do not export the Mixins type from @mui/material/styles, leading to developers needing to declare module for it from a private path: @mui/material/styles/createMixins.

The goal is to allow the following:

-declare module "@mui/material/styles/createMixins" {
+declare module "@mui/material/styles" {
   interface Mixins {
     myCustomGuttersMixin: CSSProperties;
   }
 }

[mui-bot]

No bundle size changes

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