Problem:
Processing non-fast events during SystemObj:wait() may cause two pieces
of code to interfere with each other, and is different from jobwait().
Solution:
Don't process non-fast events during SystemObj:wait().
feat(lua): add vim.system()
Problem:
Handling system commands in Lua is tedious and error-prone:
- vim.fn.jobstart() is vimscript and comes with all limitations attached to typval.
- vim.loop.spawn is too low level
Solution:
Add vim.system().
Partly inspired by Python's subprocess module
Does not expose any libuv objects.
