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)

lua/outline/types/outline.lua

@@ -93,3 +93,11 @@
 ---@class outline.OutlineOpts
 ---@field focus_outline boolean?  Whether to focus on outline of after some operation. If nil, defaults to true
 ---@field split_command string?
+
+---@class outline.BreadcrumbOpts
+---@field depth integer?
+---@field sep string?
+
+---@class outline.SymbolOpts
+---@field depth integer?
+---@field kind string?