extends the precedent set by #43703 (thanks @reflectronic) so that
`textDocument/inlayHint`, `textDocument/codeLens`, and
`textDocument/documentSymbol` correctly track multiple dynamic
registrations by id. previously each register call overwrote the slot on
`ServerCapabilities`, and unregister either silently did nothing
(inlayHint, documentSymbol — no match arm) or wiped the entire
capability (codeLens). now each method keeps its own id-keyed map on
`DynamicRegistrations`; the field on `ServerCapabilities` is cleared
only when the last registration is removed.

textDocument-sync notifications (`didChange`/`didSave`/`willSave`) are
deliberately out of scope — their fan-out semantics are ambiguous in the
spec and were flagged on @smitbarmase's earlier exploration in #36876.

verified with `cargo check -p project -p lsp -p editor`, `cargo test -p
project --test integration -- multi_registration` (3/3 passing), and
`cargo fmt -p project`.

closes part of #37838.

Release Notes:

- Improved support for language servers that dynamically register inlay
hints, code lens, or document symbols multiple times.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>