Function/Block for stopping the melody from playing

[kimprice]

Is your feature request related to a problem? Please describe. Using a conditional to switch the background melody results in a lag because the melody will continue until the end before the new melody plays. This function could also be used to stop the melody once the game over sequence begins (issue: https://github.com/microsoft/pxt-arcade/issues/1189).

Describe the solution you'd like In order to make the switch more instantaneous, we should have a function which immediately stops the melody from playing.

See the following PR for context: https://github.com/microsoft/pxt-common-packages/pull/901