This PR adds support for always on top mode, via two mechanisms:
* The global setting `alwaysOnTop`. When set to true, the window will be
created in the "topmost" group of windows. Changing this value will
hot-reload whether the window is in the topmost group.
* The action `toggleAlwaysOnTop`, which will toggle the `alwaysOnTop`
property at runtime.
## Detailed Description of the Pull Request / Additional comments
All "topmost" windows maintain an internal z-ordering relative to one
another, but they're all always above all other "non-topmost" windows.
So multiple Windows Terminal windows which are both `alwaysOnTop` will
maintain a z-order relative to one another, but they'll all be on top of
all other windows.
## Validation Steps Performed
Toggled always on top mode, both in the settings and also at runtime,
and verified that it largely did what I expected.
Closes#3038