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/symbols.lua

@@ -39,6 +39,12 @@ M.kinds = {
   [255] = 'Macro',
 }
 
+-- inverse indexing of symbols.kind
+M.str_to_kind = {}
+for k, v in pairs(M.kinds) do
+  M.str_to_kind[v] = k
+end
+
 ---@param kind string|integer
 function M.icon_from_kind(kind)
   local kindstr = kind