Instead of overlaying a half of the two-color-word character,
the two-color-word effect is now achieved
by using two half-word pseudo elements (enabled by CSS class .o),
one for the left half (::before; controlled by non-prefixed CSS classes)
and one for the right half (::after; controlled by CSS classes prefixed with 'r'),
so that the two halves of the word can be controlled independently.

Additionally, to make the text selection appear normal,
the real element is made invisible and placed on top of the pseudo elements.

This greatly reduces the number of CSS classes needed.

See Ptt-official-app/pttbbs-web#122
for an explanation of a similar implementation.

The blink effect for two-color-word characters is also implemented.

The background color was dependent on both .b<bg> and .qq<bg> classes,
which complicated the TwoColorWord blinking implementation.

Therefore, the .qq<bg> classes are removed,
while only the .qq class is left since it does not affect the background color.

Since the color of two-color-word characters is now transparent,
the color of hovered hyperlinks is now explicitly assigned as the color of q7
to prevent the link text with the two-color-word effect from being invisible.

…visual effects

This issue was described in commit a75d550
("Fix two-color character rendering bug when the right-half is black.").

This issue was caused by the HTML layout of two-color chars, e.g.:

```html
<span class="q15 b1"> <!-- Non-black BG. -->
    <span class="q15 b1 rb0 wpadding" data-text="Ａ" style="display: inline-block; width: 24px;">
        Ａ
    </span> <!-- "Black" (transparent) right-half BG., appeared as non-black -->
    Ｍ
</span>
```

To fix this issue, WordSegmentBuilder is specialized for two-color chars
(as JavaScript class TwoColorWordBuilder)
so as to instead generate such layout as:

```html
<span class="q15 b1 rb0 wpadding" data-text="Ａ" style="display: inline-block; width: 24px;">
    Ａ
</span>
<span class="q15 b1">
    Ｍ
</span>
```

This not only avoids the unwanted background color introduced by the outer span
but also eliminates the unnecessary outer span for a sole two-color char.

Note that this requires the fix introduced in commit d6ebaab
("feat: rework two-color-word effect & implement blinking")
to make the un-nested span rendered expectedly.