[theme] Rename fade to alpha

[mnajdova]

Breaking changes

• Renamed fade to alpha to better describe its functionality.

- import { fade } from '@material-ui/core/styles';
+ import { alpha } from '@material-ui/core/styles';
const classes = makeStyles(theme => ({
-  backgroundColor: fade(theme.palette.primary.main, theme.palette.action.selectedOpacity),
+  backgroundColor: alpha(theme.palette.primary.main, theme.palette.action.selectedOpacity),
}));

---

Part of #20012, the change is discussed in #13039 (comment).

[mui-pr-bot]

Details of bundle changes

Generated by 🚫 dangerJS against 9775213

fixed