Put clipboard support behind feature

[yonas]

It would be nice to compile silicon without xcb, which requires x11 and python.

[lo48576]

xcb v0.8.2 is indirectly pulled by clipboard v0.5.0, and the clipboard crate seems to be unmaintained for some years (cf. aweinstock314/rust-clipboard#91), so it sounds reasonable to make it optional or remove the dependency completely if possible.

[yonas]

Recommend switching to arclipboard - eg. https://github.com/tv42/lapce/commit/e1003d3fac00aeab66ddd08bd925f979a2ac040d