Enable selection for commands in copy-mode

[Inc0n]

This commit enables C-a and C-e to be used with shift modifer to select region to start of line and end of line, if shift-select-mode is enabled.

This is done by putting "^" into interactive, for more information see, https://www.emacswiki.org/emacs/ShiftSelectMode.

Additionally, I would recommend to bind M-w (king-ring-save) to vterm-copy-mode-done also, which is the default key bind to region save, this change I haven't introduce in this commit. I use this bind all the time, and had to manually quite copy-mode using C-c C-t, instead of using return.

I mentioned the above here, since this commit is about introducing better integration between vterm and emacs.

[Inc0n]

This pull request is transfered to #614 instead.

[Sbozzolo]

I was not aware of shift-select-mode. I don't see any reason why we shouldn't merge this PR.

[Inc0n]

Oh yes, shift-select-mode, I missed this one out (was actually not aware of this either), so this commit would be adding support for shift-select-mode. I have edited that into my first comment.