[RFC] Shell: support bell and buffer incomplete UTF-8 sequences

[bfredl]

actually fixes #4338. I will take a peek at gvim source code if there is other low hanging fruit. But more complex stuff (like colors) should be punted until :terminal is integrated.

I also merged bang_filter_spec into output_spec as in fact only tests bang. Filter is tested elsewhere.

[bfredl]

Actually our execute('!ls') behavior is consistent with gvim which is inconsistent with terminal vim.
gvim does one more thing: it makes sure to buffer an incomplete multibyte character, we should do that as well.

[janlazo]

@bfredl Can you enable the tests from bang_filter_spec.lua on Windows? I did it in janlazo@6e077b2#diff-8cc34a316e32ee95eb86a21b308e7082

[bfredl]

Thanks, I cherry picked it.

[bfredl]

Failures looks unrelated (nodejs...), marking RFC.

[justinmk]

describe("shell command :!", function()

[bfredl]

No, that is the parent of this block. If I added it it would be echoed twice in the test description. This is just a bunch of tests that share common initialization, is there another canonical way for that?

[justinmk]

My mistake, it looked like the same indent level as line 80. I would normally put it in its own scope but doesn't matter I guess.