Update ControlSequenceLength to handle colon VT param separators

The ControlSequenceLength function calculates the length of a VT control
sequence--used by LengthInBufferCells. SGR control sequences might have
parameters, separated by semicolons... or possibly colons. See:

microsoft/terminal#4321

This change updates ControlSequenceLength to treat colons similarly to
semicolons.

src/Microsoft.PowerShell.ConsoleHost/host/msh/ConsoleControl.cs

@@ -2654,7 +2654,7 @@ internal static void SetConsoleTextAttribute(ConsoleHandle consoleHandle, WORD a
         //     CSI params? '#' [{}pq]        // XTPUSHSGR ('{'), XTPOPSGR ('}'), or their aliases ('p' and 'q')
         //
         // Where:
-        //     params: digit+ (';' params)?
+        //     params: digit+ ((';' | ':') params)?
         //     CSI:     C0_CSI | C1_CSI
         //     C0_CSI:  \x001b '['            // ESC '['
         //     C1_CSI:  \x009b
@@ -2699,7 +2699,7 @@ internal static int ControlSequenceLength(string str, ref int offset)
             {
                 c = str[offset++];
             }
-            while ((offset < str.Length) && (char.IsDigit(c) || c == ';'));
+            while ((offset < str.Length) && (char.IsDigit(c) || (c == ';') || (c == ':')));
 
             // Finally, handle the command characters for the specific sequences we
             // handle: