127: Improve folding r=matklad a=aochagavia
I was messing around with adding support for multiline comments in folding and ended up changing a bunch of other things.
First of all, I am not convinced of folding groups of successive items. For instance, I don't see why it is worthwhile to be able to fold something like the following:
```rust
use foo;
use bar;
```
Furthermore, this causes problems if you want to fold a multiline import:
```rust
use foo::{
quux
};
use bar;
```
The problem is that now there are two possible folds at the same position: we could fold the first use or we could fold the import group. IMO, the only place where folding groups makes sense is when folding comments. Therefore I have **removed folding import groups in favor of folding multiline imports**.
Regarding folding comments, I made it a bit more robust by requiring that comments can only be folded if they have the same flavor. So if you have a bunch of `//` comments followed by `//!` comments, you will get two separate fold groups instead of a single one.
Finally, I rewrote the API in such a way that it should be trivial to add new folds. You only need to:
* Create a new FoldKind
* Add it to the `fold_kind` function that converts from `SyntaxKind` to `FoldKind`
Fixes#113
Co-authored-by: Adolfo Ochagavía <github@adolfo.ochagavia.xyz>
Implements a pretty barebones function signature help mechanism in
the language server.
Users can use `Analysis::resolve_callback()` to get basic information
about a call site.
Fixes#102
