Use Kernel.exec instead of system

[fschwahn]

Currently, when ending the watch process, something like this is printed:

^Crails aborted!
Interrupt: 
/Users/fabian/code/rails/app/bin/rails:5:in `<top (required)>'
/Users/fabian/code/rails/app/bin/spring:10:in `block in <top (required)>'
/Users/fabian/code/rails/app/bin/spring:7:in `<top (required)>'
Tasks: TOP => dartsass:watch
(See full trace by running task with --trace)

When using Kernel.exec the rake process is replaced with the dartsass process, so the signal goes directly to dartsass, properly ending the process.

I adopted this from webpacker, see https://github.com/rails/webpacker/blob/14dcce2c23a207062e0fc4ebed6042381dabd66a/lib/webpacker/dev_server_runner.rb#L87-L89

[fschwahn]

In light of rails/tailwindcss-rails#189 I'm going to close this.