Add external command callbacks about elvish HOT 2 OPEN

Having slept on the idea I've decided there should be a $before-external-cmd and $after-external-cmd pair of callback lists.

In the before variant each callback in the list is called with a list composed of the full path to the command, the original command name, and all arguments. If it outputs $true the next callback, if any, is invoked or the command is run. If it outputs $false no remaining callbacks are run (including the "after" callbacks) and the external command is not run. The idea is that this allows a $before-external-cmd callback to substitute different behavior for the external command. Possibly via emulation using only Elvish builtins or via some other external command (which itself is subject to the callbacks). Care needs to be taken to avoid infinite recursion. At a minimum a sane limit on callback recursion needs to be imposed by Elvish itself.

In the after variant each callback in the list is called with the time in fractional seconds the external command ran to completion, its numeric exit status, the full path to the command, the original command name, and all arguments. The after variant is primarily meant for logging which external commands are run but there may be other uses.

from elvish.

The callbacks should include additional info to make it easier to know the context in which the external command is being run; e.g., which module, script, or interactive context invoking the external command. This came to light in response to a change proposed by @xiaq to my PR #1680 to replace some external commands in the epm module with builtins.

from elvish.