Buffer stderr and stdout before triggering callbacks

[kjughx]

Currently, whenever the compile_command outputs data on stdout or stderr the corresponding on_stdout or on_stderr is called. This results in multiple (at least 5 for even one compilation error) calls to those functions which in turn do some work parsing errors. Adding stdout_buffered and stderr_buffered to vim.fn.jobstart will wait until they finish writing to those descriptors, reducing the calls to on_stdout and on_stderr to at most one.

[ej-shafran]

What about, say, a command like echo 'hello world'; sleep 10; echo 'goodbye'? Ideally, this command would write out "hello world", then wait ten seconds, then write out "goodbye". With stdout_buffered, will this still work as expected?

[ej-shafran]

As you can see, I've added a test to cover the aforementioned scenario. The test is failing on this branch, but succeeding in main. Since this pull request breaks the expected behavior, I'm closing it.

However, please note that the :Compile and :Recompile commands can be run synchronously (i.e. the output is only put in the compilation buffer once the entire command has finished):

With a bang (:Compile!), the compilation buffer will only be updated
once the command finishes (i.e. the compilation command will be run
synchronously).

(From the docs)