FAB missing exited property

[mgr34]

FAB's can have the optional ability to exit. MDC-Web supplies us with class .mdc-fab--exited [0] This class "animates the FAB out of view. When this class is removed, the FAB will return to view." What I am proposing is that FAB has a boolean prop exited. This can be used to add the class and works like other optional bool prop mini. (and I have a PR ready nearly ready)

[0] https://github.com/material-components/material-components-web/tree/v0.41.0/packages/mdc-fab#css-classes