LSP: support outline feature

[lf-]

Problem:
I want to navigate large buck files quickly and easily, so I want to be able to quickly jump to a target in the file without having to scroll through it or use ctrl-f.

Possible solution:
Implement textDocument/documentSymbol request for BUCK files as included targets, implement it for bzl files for defined symbols.

https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_documentSymbol