feat(runtime): add setup option for agnostic runtime support

[jgero]

By moving the runtime to a config option which is set during setup the
runtime parameter is not necessary anymore for each LSP cmd.

Closes #84

Also rename the global configuration variable to something plugin
related to make it obvious which plugin is responsible for that variable
and to avoid possible name collisions.

[jgero]

Should the networking mode be also contained as an setup variable? When we then create the docker/podman command the network parameter could just be a boolean to switch between none and the mode passed during setup.

@erikreinert

Setup would then look something like:

require("lspcontianers").setup({runtime = "podman", network = "slirp4netns"})

And LSP config:

require("lspconfig").gopls.setup({
  on_attach = on_attach,
  capabilities = capabilities,
  cmd = require("lspcontainers").command("gopls", {wantNetwork = true})
})

[erikreinert]

To be included in #92