syntax: Track parent LanguageLayer IDs

This commit adds a `parent` field to the `LanguageLayer`. This
information is conveniently already available when we parse injections.
This will be used in the child commit to create a type that can
traverse injection layers using this information.

helix-core/src/syntax.rs

@@ -1090,6 +1090,7 @@ impl Syntax {
  start_point: Point::new(0, 0),
  end_point: Point::new(usize::MAX, usize::MAX),
  }],
+ parent: None,
  };
 
  // track scope_descriptor: a Vec of scopes for item in tree
@@ -1360,6 +1361,7 @@ impl Syntax {
  depth,
  ranges,
  flags: LayerUpdateFlags::empty(),
+ parent: Some(layer_id),
  };
 
  // Find an identical existing layer
@@ -1525,6 +1527,7 @@ pub struct LanguageLayer {
  pub ranges: Vec<Range>,
  pub depth: u32,
  flags: LayerUpdateFlags,
+ parent: Option<LayerId>,
 }
 
 /// This PartialEq implementation only checks if that