Support mapping an operator (`g@` and `operatorfunc`)

[natetarrh]

From :help map-operator:

g@{motion}		Call the function set by the 'operatorfunc' option.
			The '[ mark is positioned at the start of the text
			moved over by {motion}, the '] mark on the last
			character of the text.
			The function is called with one String argument:
			    "line"	{motion} was |linewise|
			    "char"	{motion} was |characterwise|
			    "block"	{motion} was |blockwise-visual|

Looking into what a proper surround.vim port would take, this is necessary for defining and applying the "surround" operator.

[nightwing]

The problem is that we do not have support for defining functions to use with operatorfunc.

Are you trying to use surround.vim implemented in vim script? It may be easier to implement it in javascript instead of adding vimscript support.