This PR adds a new command to the Carbon VSCode extension "carbon:
Restart language server" which acts similar to clangd / bazel
extensions. While not useful during regular working of the extension
this helps when LSP either crashes or the toolchain is updated.
In order to support graceful restart, I needed to add a Call handler for
"shutdown" which is noop for now. Lsp clients always call shutdown
before sending exit.
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Co-authored-by: jonmeow <jperkins@google.com>
DavidLoftusandjonmeow