Add ide-specific test-harness packages

[pokey]

We are currently abusing the main field in packages/test-harness/package.json and pretending it's neovim-specific in order to ease testing locally and in CI. We should try to create a neovim-test-harness package, as well as packages for other IDEs, and I guess one generic one for things like unit tests. Talon tests might also need their own package?

Originally posted by @pokey in #2256 (comment)

[saidelike]

Could we have a generic entry point and abuse that main field still but have an environment variable (e.g. APP_NAME=neovim) that allows dispatching to the ide-specific test method (neovim, etc.) instead of having to create yet another package?