20210404-111518-fe48951e

macos: fixup SUPER+SHIFT+[ key decoding

One of the default key assignments was registered as `SUPER+SHIFT+{`
which worked on macOS, but on Linux, would never match because the
keypress over there was (correctly) reporting as `SUPER+{`.

I originally thought that the user reported issue was a linux
normalization problem, but in looking deeper, the issue is really
that macos is doing something funky!

On macos we collect the interpreted key event as a string, and also
the interpretation of that event without any modifiers applied.

For letters this means that eg: `ALT-l` reports as `¬` for the
processed string and `l` for the unmodified string.  That's good!

However, for punctuation we get a backwards result: SUPER+SHIFT+[
produces `[` for the processed text and `{` for the unmodified
text!

This commit tries to detect this, using a heuristic that is
potentially bad on non-US layouts: if both the processed and
unmodified strings are punctuation then we bias to the unmodified
version.

With that change, that key press is correctly reported as `SUPER+{`,
and we can fix the key assignment registration to reflect that.

I quickly checked the behavior of pressing that same physical key
combination with a DEU layout active, and it appears that the unmodified
stuff is also flipped there; we get a lower-case version of something
that I think should be uppercase.  This commit doesn't change that
behavior:

```
key_event KeyEvent { key: Char('ü'), modifiers: NONE,
        raw_key: Some(Char('Ü')),
        raw_modifiers: SHIFT | SUPER,
        raw_code: Some(33),
        repeat_count: 1, key_is_down: true }
```

refs: https://github.com/wez/wezterm/issues/601