feat: support document symbols requests in lsp

[MaartenStaa]

This allows requesting a list/tree of symbols in the current document. The following items are currently exposed:

• load statements:
  • The module path
  • The loaded symbol names
• def statements:
  • The function node
  • Argument names
• Closures, when assigned to a named variable
• Structs (calls to struct), when assigned to a named variable
• Build system targets (function calls containing a name argument)

Example from starlark_syntax/testcases/parse/android_sdk_repository_template.star: