Filters in the playground

[jgm]

I've added a demonstration of "filters" that transform the AST: https://djot.net/playground/

Click the "Filter" button and choose an example, which you can edit. When you close the modal window, the filter will be applied, or, if there is an error, the window will be kept open and an error message displayed.

There is some documentation on these in https://github.com/jgm/djot/blob/main/doc/djot.md.