Add new reporter to support LSP

[david-yz-liu]

Looking ahead to support the Language Server Protocol (LSP), I'd like to create a new reporter, LSPReporter, that outputs JSON in the format specified by LSP. (LSP 3.17 specification; particularly relevant is the "Diagnostic" part.)

We should make use of the types provided by lsprotocol package to generate the output, rather than manually write out all of the JSON ourselves. (See the definitions in types.py, which should correspond to the formal specification.

Note that we aren't building a full language server here, so no need to worry about receiving requests or anything like that. This task is just about outputting an LSP-compliant response.