term: add support to catch keys directly when entered

[bytedream]

Adds functionality to catch keys / characters right after the terminal receives them.
This is very helpful when you want to handle, for example, special key combinations like Ctrl-C yourself and not just let them terminate your terminal without anything you can do about it.

To receive this, a new OnKey field was added to the terminal struct.

type Terminal struct {
    ...
    OnKey map[rune]func(key rune) (resume bool)
    ...
}

The map stores pairs of runes (which represents a keycode) and functions. Whenever a key is entered and the keycode of it exists in this map the corresponding functions gets called. The function takes the key as its only argument and returns if the normal behaviour of the key, if existent, should continue. This behaviour could be the termination of the terminal (if the key was Ctrl-C) or just a normal letter.

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[bytedream]

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This PR (HEAD: 891a970) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/term/+/358214 to see it.

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[ardnew]

Instead of inserting the check here as well as sprinkled judiciously throughout readLine (a little hard to follow), consider inserting it in one common location. I haven't verified, but immediately after checking for utf8.RuneError, around line 759, seems like it would catch all these same cases.

(I assume handleKey was the first guess at this "common location", which sounds very reasonable to me, given the name)