Playground: Add a way to clear and reset the editor text from the UI.

[DanKaplanSES]

Visiting https://www.typescriptlang.org/play loads the last code you saved. While generally useful, on mobile, this makes it challenging to create new playground: the editor does not support select all (in a way I could figure out) or drag selection, so the only way to create a new playground was the hold the delete button until all the text is gone. Given the last code I wrote, this would've required holding down the delete button for at least a minute.

Repro:

1. Using the android browser, create a playground and save it.
2. Close the tab.
3. Go back to https://www.typescriptlang.org/play
4. Try to create a new playground

Expected behavior:
Somewhere in the playground, perhaps in a menu, there is a way to clear the last saved playground.

Actual behavior:
I could only delete the text content by holding the delete button and waiting.

Workaround:
This URL will bring up a blank playground every time: https://www.typescriptlang.org/play?#code/Q. It can be bookmarked to achieve the same effect.

I have not tested this on any iOS device.

[typescript-bot]

Hello! As per #2804, we are automatically closing all open issues. Please see #2804 for a description of what issues and PRs can be accepted going forward.