feat: implement getter and setter for CUTBUFFER (For example, interaction with the system clipboard)

[MuXiu1997]

I'm considering wrapping the CUTBUFFER into getter and setter functions. This approach might minimize the extent of changes required in the existing code while also allowing other users to override these two functions to fulfill their own needs, such as interacting with the system clipboard.

For example:

# CUTBUFFER getter
function zvm_get_cutbuffer() {
  pbpaste
}

# CUTBUFFER setter
function zvm_set_cutbuffer() {
  echo -n $1 | pbcopy
}

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• Close #216

[MuXiu1997]

@jeffreytse I hope to receive your advice and response.