Support parallel rspec tests

[lawrencegripper]

This PR adds support for using parallel_rspec to the test adapter.

The main changes require where:

1. Allow a slightly different command syntax to be passed
2. Instead of receiving just one START_OF_TEST_JSON line I'd get 1 per parallel execution and need to merge them all back together.

Note the parallel test suite partitions by file so it only really makes sense to use this when the whole suite is being run.

For the codebase I work on this made it go from 7mins to 1min to execute all tests :partying_face:

CI Action here is :heavy_check_mark: https://github.com/lawrencegripper/vscode-ruby-test-adapter/actions/runs/5015831721

Here it is in action :point_down:

Kazam_screencast_00010.webm