embed jsonrpc types from jsonrpc-core crate (#2801)

We should not depend on jsonrpc-core anymore:

* The project just announced it's no longer actively maintained[^1],
  preferring their new implementation in `jsonrpsee`.
* The types are too strict: we would benefit from removing some
  `#[serde(deny_unknown_fields)]` annotations to allow language
  servers that disrespect the spec[^2].
* We don't use much of the project. Just the types out of core.
  These are easy to embed directly into the `helix-lsp` crate.

[^1]: https://github.com/paritytech/jsonrpc/pull/674
[^2]: https://github.com/helix-editor/helix/issues/2786

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