chore(snack-bar): rename MdSnackBarRef instance to componentInstance

[shaunak1111]

BREAKING CHANGE: Renamed variable instance to componentInstance to be consistent with other dialogs

Closes #6252

[josephperrott]

LGTM

[kara]

Note: internal updates required

[josephperrott]

Since this didn't make it in before RC, we will need to mark instance as deprecated have it still available to be used as a reference for the time being.

You should be able to just have a getter so the value is available:

/**
 * @docs-private
 * @deprecated
 */
get instance() { return this.componentInstance; }

[jelbourn]

Closing this since, after releasing 5.0.0, we aren't making any more breaking API changes.

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