Add the ability to show the tab bar in fullscreen (#18171)

This PR allows users to enable the tab bar in fullscreen mode.

A new setting; "showTabsFullscreen"; has been added which accepts a
boolean value. When `true`, then the tab bar will remain visible when
the terminal app is fullscreen. If the value is `false` (default), then
the tab bar is hidden in fullscreen.

When the tab bar is visible in fullscreen, the min/max/close controls
are hidden to maintain the expected behaviour of a fullscreen app.

## Validation Steps Performed
All unit tests are passing.

Manually verified that when the "launchMode" setting is "fullscreen" and
the "showTabsFullscreen" setting is `true`, the tab bar is visible on
launch.

Manually verified that changing the setting at runtime causes the tab
bar to be shown/hidden immediately (if the terminal is currently
fullscreen).

Manually verified that the new "showTabsFullscreen" setting is honoured
regardless of whether "showTabsInTitlebar" is set to `true` or `false`.

Closes #11130

doc/cascadia/profiles.schema.json

@@ -2532,6 +2532,11 @@
           "description": "When set to true, the Terminal's tab row will display a shield icon when the Terminal is running with administrator privileges",
           "type": "boolean"
         },
+        "showTabsFullscreen": {
+          "default": false,
+          "description": "When set to true, tabs remain visible in fullscreen mode. When set to false, tabs will be hidden when entering fullscreen mode.",
+          "type": "boolean"
+        },
         "useAcrylicInTabRow": {
           "default": false,
           "description": "When set to true, the tab row will have an acrylic material background with 50% opacity.",