Add textDocument/documentSymbol support

[Jason-Abbott]

Implement document symbols to provide an outline/symbol tree for templates.

Symbols to expose:

• Template definitions: {{define "name"}} and {{block "name"}}
• Variable declarations: $var := ...
• Control flow blocks: if, range, with (as a hierarchy)

Symbol kinds (LSP SymbolKind):

• Template definitions → Function (12) or Module (2)
• Variables → Variable (13)
• Control flow → Struct (23) or Namespace (3)

Implementation notes:

• Walk AST and collect symbols with their ranges
• Preserve hierarchy (variables inside templates, etc.)
• Include symbol details like template parameter type if known

LSP methods:

• textDocument/documentSymbol - return symbol hierarchy