Allow both integer and string request IDs in LSP (#7662)

Zed's LSP support expects request messages to have integer ID, Metals
LSP uses string. According to specification, both is acceptable:

interface RequestMessage extends Message {

	/**
	 * The request id.
	 */
	id: integer | string;

...
This pull requests modifies the types and serialization/deserialization
so that string IDs are accepted.

Release Notes:

- Make Zed LSP request ids compliant to the LSP specification