add footer to command-pal with configurable text

[rouilj]

New CommandPal option footerText. If set to null (default) no footer is displayed. If set to a string the string is displayed.

The way I handle conditionally showing the footer is kind of ugly. Svelte doesn't have conditional slots. Once:

https://github.com/sveltejs/svelte/issues/5604

is implemented, the whole mess can be cleanly rewritten.

I would have like to use :where(.footer) in the css for styling. But there are a lot of mobile browsers on caniuse that have unknown status even 2 years after their release.

Using :where would allow ".footer" in user's css to work. As it is you have to make ".footer" more specific using e.g. "#id .footer" or ".footer[slot=footer]".

README.md, and cp-advanced files updated to show/use.