yaml: improve highlights and locals (#946)

- More scopes
- Highlight strings as strings
- Recognize more keys/fields

Improvements can be seen with this weird yaml:

```yaml
{ foo: bar }
---
{a: [b, c], [d, e]: f}

---
- { single line, a: b}
- { multi
  line, a: b}
---

"fooo": bar
'foo': bar
one: two
```

queries/yaml/locals.scm

@@ -1,4 +1,8 @@
-(document) @scope
+[
+ (stream)
+ (document)
+ (block_node)
+] @scope
 
 (anchor_name) @definition
 (alias_name) @reference