Add `feedkeys` API function

Problem: Vim has --remote-send to pass key input into a Vim server instance. This is different from what we currently have in the API because it calls replace_termcodes to replace \<CR> or \<Leader> without using backslah notation like we see in string expressions. This becomes problematic when trying to port applications that rely on the remote-send expressions (google/vroom#36).
 
The code from this commit comes straight from Vim main.c:server_to_input_buf() (plus lint fixes and necessary adjustments)

ping @tarruda