Implement lsp/formatting for Carbon::LanguageServer (#7688)

Implements formatting support within LSP. Current implementation
performs while file formatting, we just replace file with output of
Carbon::Format()

https://github.com/carbon-language/carbon-lang/pull/7687 independently
improves formatting so that this output is somewhat decent.

Assisted-With: Gemini / Antigravity

toolchain/language_server/handle_initialize.cpp

@@ -32,6 +32,7 @@ auto HandleInitialize(
 
   llvm::json::Object capabilities{{"declarationProvider", true},
                                   {"definitionProvider", true},
+                                  {"documentFormattingProvider", true},
                                   {"documentSymbolProvider", true},
                                   {"hoverProvider", true},
                                   {"positionEncoding", encoding},