This PR updates the rendering of the underlined graphic rendition
attribute, using the style specified in the active font, instead of just
reusing the grid line at the bottom of the character cell.

• Support for drawing the correct underline effect in the grid line
  renderer was added in Refactor grid line renderers with support for more line types #7107.

There was already an ExtendedAttributes flag defined for the
underlined state, but I needed to update the SetUnderlined and
IsUnderlined methods in the TextAttribute class to use that flag
now in place of the legacy LVB_UNDERSCORE attribute. This enables
underlines set via a VT sequence to be tracked separately from
LVB_UNDERSCORE grid lines set via the console API.

I then needed to update the Renderer::s_GetGridlines method to
activate the GridLines::Underline style when the Underlined
attribute was set. The GridLines::Bottom style is still triggered by
the LVB_UNDERSCORE attribute to produce the bottom grid line effect.

Validation

Because this is a change from the existing behaviour, certain unit tests
that were expecting the LVB_UNDERSCORE to be toggled by SGR 4 and
SGR 24 have now had to be updated to check the Underlined flag
instead.

There were also some UI Automation tests that were checking for SGR 4
mapping to LVB_UNDERSCORE attribute, which I've now substituted with a
test of the SGR 53 overline attribute mapping to
LVB_GRID_HORIZONTAL. These tests only work with legacy attributes, so
they can't access the extended underline state, and I thought a
replacement test that covered similar ground would be better than
dropping the tests altogether.

As far as the visual rendering is concerned, I've manually confirmed
that the VT underline sequences now draw the underline in the correct
position and style, while grid lines output via the console API are
still displayed in their original form.

Closes #2915