derive capabilities version from client version

The client reports client capabilities protocol version. See here: https://github.com/redhat-developer/rsp-client/blob/master/src/client.ts#L116

export class RSPClient {
    ...
    /**
     * Returns the capabilities implemented by the client
     */
    getCapabilities(): Protocol.ClientCapabilitiesRequest {
        return {map: {'protocol.version': '0.16.0', 'prompt.string': 'true'}};
    }

This version should match the client/protocol version and tends to be forgotten when bumping the client version. We thus should find a way to derive this version from the client version.
A suggestion is to parse package.json, extract the version and use it when building the client. See here: https://github.com/redhat-developer/rsp-client/pull/87#issuecomment-499432884

Start in src/client.ts at RSPClient.getCapabilities() and review package.json for the client version. Check the approach suggested in pull request 87's comment, then verify that the reported protocol.version is derived from the package version and remains synchronized when the client version changes.