[material-ui] Update mergeSlotProps to merge style

[siriwatknp]

Continue from #44809, this PR merge the style instead of replacing between default slot props and external slot props.

I think it's more common from user perspective to have style merged rather than replace.

Some of the Material UI components, e.g. SwipeableDrawer, also merge the style. So this PR will benefit both internal and users.

If the user wants to remove some style from the default style, they can provide null as a value:

// default `pointerEvents` is "none"
<CustomSwipeableDrawer slotProps={{ paper: { style: { pointerEvents: null } }}>

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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• docs/data/material/guides/composition/composition.md

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