Add r.runSelectionRetainCursor

[renkun-ken]

What problem did you solve?

Closes #324

This PR adds a new command r.runSelectionRetainCursor that runs selection but does not move cursor.

(If you do not have screenshot) How can I check this pull request?

Run command r.runSelectionRetainCursor when cursor is at the first line of the following code:

m <- rnorm(100)
n <- rnorm(100)

then the first line should be sent to terminal but the cursor does not move.

It should also work with multi-line expression. Run when cursor is at first of last line:

m <- local({
  x <- rnorm(100)
  y <- rnorm(100)
})

Both should send the entire expression but cursor should not move.

[renkun-ken]

I set up the key binding here but I'm not sure why it does not come to be my default key bindings when I run the extension.

[andycraig]

For me alt+enter worked without needing to change anything. Maybe you have something else bound to alt+enter that has higher priority?

[andycraig]

LGTM. Tested on Windows.

[renkun-ken]

Thanks for testing!