[theme] Remove hardcoded breakpoints

There are several places in the codebase where breakpoints are hardcoded.
Since we have the ability to override breakpoints, we should not have hardcoded values in the code

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

1. PropTypes

PropTypes.oneOf(['xs', 'sm', 'md', 'lg', 'xl']),
PropTypes.arrayOf(PropTypes.oneOf(['xs', 'sm', 'md', 'lg', 'xl'])),

2. Styles

[theme.breakpoints.only('xs')]: {
  ...
}

3. Functions in withWidth.js

import { breakpointKeys } from '../styles/createBreakpoints';
function isWidthUp(...){ 
  return breakpointKeys.indexOf(...)
}
function isWidthDown(...){ 
  return breakpointKeys.indexOf(...)
}

Expected Behavior 🤔

1. PropTypes

PropTypes.string
PropTypes.arrayOf(PropTypes.string),

2. Styles

a) Use a media query only if there is such a breakpoint in the theme

[theme.breakpoints.only(theme.breakpoints.keys['xs'] ?? 0)]: {
  ...
}

b) Add a new property alias to createBreakpoints function to match breakpoints

createBreakpoints({
  values: {
      tablet: 640,
      laptop: 1024,
      desktop: 1280,
    },
    alias: {
      xs: 'tablet',
      sm: 'tablet',
    },
  }
})

3. Functions in withWidth.js
   a) Show the necessity for breakpoints
   Remove import { breakpointKeys } from '../styles/createBreakpoints';

function isWidthUp(breakpoints, breakpoint, width, inclusive = true){ .. }

b) Add breakpoints to options implicitly

import { breakpointKeys } from '../styles/createBreakpoints'
function isWidthUp(breakpoint, width, opts = {}) {
  let {breakpoints = breakpointKeys, inclusive = true } = opts;
  ...
}

Your Environment 🌎

Tech	Version
Material-UI	v4.11.0

Start by reading styles/createBreakpoints and withWidth.js, then locate the listed PropTypes and style usages that hardcode xs through xl. Compare the proposed theme override and alias approaches before choosing a consistent direction. Done means breakpoint validation, media-query styles, and withWidth helpers no longer assume fixed breakpoint names.