Adds support for setting the cursor visibility in Terminal. Visibility
is a property entirely independent from whether the cursor is "on" or
not. The cursor blinker _should_ change the "IsOn" property. It was
actually changing the "Visible" property, which was incorrect. This PR
additionally corrects the naming of the method used by the cursor
blinker, and makes it do the right thing.
I added a pair of tests, one taken straight from conhost. In
copy-pasting that test, I took it a step further and implemented
`^[[?12h`, `^[[?12l`, which enables/disables cursor blinking, for the
`TerminalCore`. THIS DOES NOT ADD SUPPORT FOR DISABLING BLINKING IN THE
APP. Conpty doesn't emit the blinking on/off sequences quite yet, but
when it _does_, the Terminal will be ready.
## References
* I'd bet this conflicts with #2892
* This isn't a solution for #1379
* There shockingly isn't an issue for cursor blink state via conpty...?
## PR Checklist
* [x] Closes#3093
* [x] Closes#3499
* [x] Closes#4644
* [x] I work here
* [x] Tests added/passed
* [n/a] Requires documentation to be updated
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