lsp: Offer to populate empty documents

[hunger]

So when snippets do not work: Let's try something else to get started from scratch:-)

[hunger]

Of course this only shows while the file does not contain anything but whitespace:-)

[hunger]

We need to remember which area of the document we are actually touching... otherwise this just can not work.

[hunger]

The latest set of commits add an ASCII start-of-text and end-of-text byte around the area covered by slint! macros.

It then limits the actions to the range between these markers (if present). That way it works with the slint! macro

BUT there is a bug still: When you remove the slint macro, the LSP will not update its document cache accordingly and will insist that the code lense needs to continue to exist. That needs fixing :-/