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/highlights.scm

@@ -1,9 +1,9 @@
-(block_mapping_pair key: (_) @field)
-(flow_mapping (_ key: (_) @field))
 (boolean_scalar) @boolean
 (null_scalar) @constant.builtin
 (double_quote_scalar) @string
 (single_quote_scalar) @string
+(block_scalar) @string
+(string_scalar) @string
 (escape_sequence) @string.escape
 (integer_scalar) @number
 (float_scalar) @number
@@ -14,20 +14,35 @@
 (yaml_directive) @keyword
 (ERROR) @error
 
+(block_mapping_pair
+  key: (flow_node [(double_quote_scalar) (single_quote_scalar)] @field))
+(block_mapping_pair
+  key: (flow_node (plain_scalar (string_scalar) @field)))
+
+(flow_mapping
+  (_ key: (flow_node [(double_quote_scalar) (single_quote_scalar)] @field)))
+(flow_mapping
+  (_ key: (flow_node (plain_scalar (string_scalar) @field))))
+
 [
-","
-"-"
-":"
-">"
-"?"
-"|"
+ ","
+ "-"
+ ":"
+ ">"
+ "?"
+ "|"
 ] @punctuation.delimiter
 
 [
-"["
-"]"
-"{"
-"}"
+ "["
+ "]"
+ "{"
+ "}"
 ] @punctuation.bracket
 
-["*" "&"] @punctuation.special
+[
+ "*"
+ "&"
+ "---"
+ "..."
+] @punctuation.special

queries/yaml/locals.scm

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