feat(api): Get symbol and breadcrumb at location

Currently the implementation is very limited.

Ref: #24

- Outline must be open and have been loaded for it to work (requires
  lazy loading or headless loading of Sidebar)
- Empty string returned if cursor is not in any symbol ('closest' symbol
  not yet supported)
- Line column not used
- Returning concatenated symbol names rather than a list of tables with
  node info (requires a refactor of outline.SymbolNode type)
- Subject to config.symbols.filter and folds (requires finding hover
  list somewhere outside of writer.make_outline)

README.md

@@ -790,6 +790,25 @@ require'outline'
   This is automatically called on events
   `outline_items.auto_update_events.refresh` from config.
 
+- **get_breadcrumb(opts)**
+
+  Return a string concatenated from hovered symbols hierarchy representing code
+  location.
+
+  Optional opts table fields:
+  - depth (nil): Maximum depth of the last symbol included. First item has depth 1.
+    Set to 0 or nil to include all
+  - sep (` > `): String for separator
+
+- **get_symbol(opts)**
+
+  Return the symbol name of the deepest hovered symbol representing code
+  location.
+
+  Optional opts table fields:
+  - depth (nil): Maximum depth of the returned symbol
+  - kind (nil): Symbol kind to search for (string).
+
 
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