Used for adding imports to the file when completing.

Zero-width matches at the start of the file make no sense to us.

Can't do it via a scm query nicely because it returns an iterator over
all the matches, whereas ...

We only rely on some of the rendering primitives and implement our
Cursive-style compositor on top.

From:
{|}

To:
{
|
}

Blocked on https://github.com/NixOS/nix/issues/4423, build doesn't see
submodules.

The default handler is now simply a fallback.

This way edit -> undo will properly show up as unmodified.

Still need to implement the clangd encoding negotiation, but it's
a start. Should also manually ...