Fix inconsistent return value of request function

[k-86]

When lspEngine==nvim-lsp && opt.sync==true, the request function in request.ts returns response: {err?: lsp.ResponseError, result: unknown}. Otherwise, it returns response.result: unknown.

By unifying the return value to response.result: unknown, Source.#resolve will work when lspEngine==lspoints.

Note: When lspEngine==vim-lsp, Source.#resolve does not work due to another problem.

[Shougo]

Thanks.

[Shougo]

It does not fix #61 problem?