feat: Report document symbols of kind variable for let statements

[LastExceed]

initially i had implemented a bunch more kinds, but eventually realized that ide_db::SymbolKind != lsp_types::SymbolKind and that many of the former kind are mapped to the same latter kind, which could cause annoyances when e.g. the user wants their outline view in vscode to show ide_db::SymbolKind::Const and ide_db::SymbolKind::Static but not ide_db::SymbolKind::ConstParam (which wouldn't be possible to configure in vscode since all of them are mapped to lsp_types::SymbolKind::CONSTANT) so i decided to take a conservative approach and ended up removing all but 1 kind

[Veykril]

What is the goal here? We generally don't consider let statements to be part of the file structure

[LastExceed]

the goal is to make as much language agnostic information accessible as possible. The client can always choose to selectively ignore stuff. E.g. if you don't want variable declarations to show up in VSCode's outline view then you can simply uncheck Settings > Outline > Show Variables

My personal motivation (which brought me here in the first place) is a language agnostic extension I am developing which selectively fades everything outside the currently selected scope/expression/whatever to reduce visual noise. This extension is currently more capable in e.g. TypeScript than it is in Rust simply because the TS language server provides more information than RA does

Whether or not variable declarations are part of the file structure is IMO a matter of opinion/preference, and I see no good reason to withhold information when configuration options exist

[Veykril]

Fair, I think the last time someone proposed adding matches to this which was confusing.

[Veykril]

Please add to the test_file_structure test

[LastExceed]

sry for the delay. added tests and rebased the branch