Text underline cannot be retextured

Relates to: MC-268805 MC-268927 MC-269079 MC-269364 MC-269365 MC-269437 MC-269438 MC-269443 MC-269447 MC-269938 MC-269939 MC-270254 MC-270255 MC-270256 MC-270257 MC-270258 MC-270260 MC-270324 MC-270766 MC-273060 MC-275778 MC-276442 MC-276635 MC-276664 MC-277504 MC-277696 MC-277705 MC-277706 MC-277877

The bug

The underline displayed under text does not appear to be tied to a texture, unlike almost all other aspects of fonts.

Example underlined text

/tellraw @p {"text": "\u0020", "underlined": true}

Expected results

Underlines would have a texture file or some other configuration method such that the way it looks can be customized in resource packs.

Actual results

They do not.