Add editing langset feature.

Add editing langset feature.

The syntax to add any langset to the langset table looks like:

<match target="scan">
    <test name="family">
        <string>Buggy Sans</string>
    </test>
    <edit name="lang" mode="assign">
        <plus>
            <name>lang</name>
            <langset>
                <string>zh-cn</string>
                <string>zh-tw</string>
            </langset>
        </plus>
    </edit>
</match>

To remove any langset from the langset table:

<match target="scan">
    <test name="family">
        <string>Buggy Sans</string>
    </test>
    <edit name="lang" mode="assign">
        <minus>
            <name>lang</name>
            <langset>
                <string>ja</string>
            </langset>
        </minus>
    </edit>
</match>