[TTX] Add AxisNameID value to AxisIndex as comment

[davelab6]

I have a 3 axis font, Weight Grade Roundness, per https://github.com/googlefonts/axisregistry, and reading the TTX of the STAT table, I see

  <STAT>
    <Version value="0x00010001"/>
    <DesignAxisRecordSize value="8"/>
    <!-- DesignAxisCount=3 -->
    <DesignAxisRecord>
      <Axis index="0">
        <AxisTag value="wght"/>
        <AxisNameID value="256"/>  <!-- Weight -->
        <AxisOrdering value="0"/>
      </Axis>
      <Axis index="1">
        <AxisTag value="GRAD"/>
        <AxisNameID value="257"/>  <!-- Grade -->
        <AxisOrdering value="1"/>
      </Axis>
      <Axis index="2">
        <AxisTag value="ROND"/>
        <AxisNameID value="268"/>  <!-- Roundness -->
        <AxisOrdering value="2"/>
      </Axis>
    </DesignAxisRecord>
    <!-- AxisValueCount=11 -->
    <AxisValueArray>
      <AxisValue index="0" Format="1">
        <AxisIndex value="0"/>
        <Flags value="0"/>
        <ValueNameID value="259"/>  <!-- Thin -->
        <Value value="100.0"/>
      </AxisValue>
      <AxisValue index="1" Format="1">
        <AxisIndex value="0"/>
        <Flags value="0"/>
        <ValueNameID value="260"/>  <!-- ExtraLight -->
        <Value value="200.0"/>
      </AxisValue>
      <AxisValue index="2" Format="1">
        <AxisIndex value="0"/>
        <Flags value="0"/>
        <ValueNameID value="261"/>  <!-- Light -->
        <Value value="300.0"/>
      </AxisValue>
      <AxisValue index="3" Format="3">
        <AxisIndex value="0"/>
        <Flags value="2"/>  <!-- ElidableAxisValueName -->
        <ValueNameID value="2"/>  <!-- Regular -->
        <Value value="400.0"/>
        <LinkedValue value="700.0"/>
      </AxisValue>
      <AxisValue index="4" Format="1">
        <AxisIndex value="0"/>
        <Flags value="0"/>
        <ValueNameID value="262"/>  <!-- Medium -->
        <Value value="500.0"/>
      </AxisValue>
      <AxisValue index="5" Format="1">
        <AxisIndex value="0"/>
        <Flags value="0"/>
        <ValueNameID value="263"/>  <!-- SemiBold -->
        <Value value="600.0"/>
      </AxisValue>
      <AxisValue index="6" Format="1">
        <AxisIndex value="0"/>
        <Flags value="0"/>
        <ValueNameID value="264"/>  <!-- Bold -->
        <Value value="700.0"/>
      </AxisValue>
      <AxisValue index="7" Format="1">
        <AxisIndex value="0"/>
        <Flags value="0"/>
        <ValueNameID value="265"/>  <!-- ExtraBold -->
        <Value value="800.0"/>
      </AxisValue>
      <AxisValue index="8" Format="1">
        <AxisIndex value="0"/>
        <Flags value="0"/>
        <ValueNameID value="266"/>  <!-- Black -->
        <Value value="900.0"/>
      </AxisValue>
      <AxisValue index="9" Format="1">
        <AxisIndex value="1"/>
        <Flags value="2"/>  <!-- ElidableAxisValueName -->
        <ValueNameID value="267"/>  <!-- Normal -->
        <Value value="0.0"/>
      </AxisValue>
      <AxisValue index="10" Format="1">
        <AxisIndex value="2"/>
        <Flags value="2"/>  <!-- ElidableAxisValueName -->
        <ValueNameID value="269"/>  <!-- Default -->
        <Value value="0.0"/>
      </AxisValue>
    </AxisValueArray>
    <ElidedFallbackNameID value="2"/>  <!-- Regular -->
  </STAT>

In the <AxisValue> records, it would be more readable for me to add the comment next to the <AxisNameID> elements near the top also to the <AxisIndex> elements later on, so I don't have to see the <AxisIndex> value and look it up in the <Axis> element value.

So eg that last one would go from

      <AxisValue index="10" Format="1">
        <AxisIndex value="2"/>
        <Flags value="2"/>  <!-- ElidableAxisValueName -->
        <ValueNameID value="269"/>  <!-- Default -->
        <Value value="0.0"/>
      </AxisValue>

to

      <AxisValue index="10" Format="1">
        <AxisIndex value="2"/> <!-- Roundness -->
        <Flags value="2"/>  <!-- ElidableAxisValueName -->
        <ValueNameID value="269"/>  <!-- Default -->
        <Value value="0.0"/>
      </AxisValue>