Add `identifier` to pull diagnostic LSP capabilities

This field in the server capabilities instructs the client to maintain
the diagnostics received from a `textDocument/diagnostic` pull request
as a separate set from other diagnostics: namely those sent with classic
"push" diagnostics, `textDocument/publishDiagnostic`. rust-analyzer
emits "native" diagnostics (computed by rust-analyzer itself) in pull
diagnostics and separately emits cargo-based diagnostics with push, so
push and pull diagnostics should be different sets. Setting this field
instructs the client to avoid clearing push diagnostics when new pull
diagnostics arrive and vice versa.

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