[css-paint-api][worklets] Edits made for FPWDs

Hi all,

I've made the edits discussed for the FPWD discussed in the F2F.
https://drafts.css-houdini.org/css-paint-api/
https://drafts.css-houdini.org/worklets/

One issue which we resolved on, allowing both "inputProperties" and
function "inputArguments" turns out to more complicated than first expected.

(See https://github.com/w3c/css-houdini-drafts/issues/100 for full details).

Basically defining a grammar to accept as arguments for the paint function
is complicated, (doesn't belong in the Paint API), and should wait once the
Parser API is more evolved.

We have a resolution to publish an FPWD of paint. Could we resolve on list
to publish FPWD of worklets as well?

Thanks,
Ian