[LinearProgress] Add more classes keys

There are some useful classes that could be easily added along with a potential class rename to avoid confusion

• This is not a v0.x issue.
• I have searched the issues of this repository and believe that this is not a duplicate.

Expected Behavior 🤔

There should be a way to configure classes for the root element when variant="determinate", variant="indeterminate" or variant="query" as well as a way to configure classes for both bar1 & bar2 elements when variant="indeterminate" or variant="query".

Example:

const styles = {
    barDeterminate: { /* May be less confusing than bar1Determinate */
        backgroundColor: "green"
    },
    barIndeterminate: { /* As an option to avoid using both bar1Indeterminate AND bar2Indeterminate */
        backgroundColor: "green"
    },
    determinate: {
        backgroundColor: "green"
    },
    indeterminate: {
        backgroundColor: "green"
    }
}

Current Behavior 😯

The above classes do nothing when passed in to the classes prop.

Examples 🌈

[oliviertassinari]

@alxsnchez Sure, you can go ahead 😃.

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