Added selectionBackground to light color schemes (#16243)

Add a selectionBackground property which is set to the scheme's
brightBlack too all 3 of the light color schemes.

Related to #8716
It does not close the bug because as mentioned in the issue, when you
input numbers, they seem to be invisible in the light color schemes and
selecting them with the cursor doesn't reveal them.

(cherry picked from commit a5c269b)
Service-Card-Id: 91033166
Service-Version: 1.18

src/cascadia/TerminalSettingsModel/defaults.json

@@ -174,6 +174,7 @@
             "foreground": "#383A42",
             "background": "#FAFAFA",
             "cursorColor": "#4F525D",
+            "selectionBackground": "#4F525D",
             "black": "#383A42",
             "red": "#E45649",
             "green": "#50A14F",
@@ -218,6 +219,7 @@
             "foreground": "#657B83",
             "background": "#FDF6E3",
             "cursorColor": "#002B36",
+            "selectionBackground": "#073642",
             "black": "#002B36",
             "red": "#DC322F",
             "green": "#859900",
@@ -262,6 +264,7 @@
             "foreground": "#555753",
             "background": "#FFFFFF",
             "cursorColor": "#000000",
+            "selectionBackground": "#555753",
             "black": "#000000",
             "red": "#CC0000",
             "green": "#4E9A06",