This PR adds support for the ANSI _crossed-out_ graphic rendition
attribute, which is enabled by the `SGR 9` escape sequence.
* Support for the escape sequences and storage of the attribute was
originally added in #2917.
* Support for drawing the strikethrough effect in the grid line renderer
was added in #7107.
Since the majority of the code required for this attribute had already
been implemented, it was just a matter of activating the
`GridLines::Strikethrough` style in the `Renderer::s_GetGridlines`
method when the `CrossedOut` attribute was set.
VALIDATION
There were already some unit tests in place in `VtRendererTest` and the
`ScreenBufferTests`, but I've also now extended the SGR tests in
`AdapterTest` to cover this attribute.
I've manually confirmed the first test case from #6205 now works as
expected in both conhost and Terminal.
Closes#6205
ef4aed944a