Make sure Terminal state machine always accepts C1 controls (#13969)

When we added support for the `DECAC1` control sequence, which
determines whether `C1` controls are accepted or not, the intention was
that conhost would be making that determination, and Windows Terminal
would always be expected to accept any passed-through `C1` controls.

However, this didn't take into account that a passed-through `RIS`
sequence could end up disabling `DECAC1`, and that would leave Windows
Terminal incapable of processing any `C1` controls. This PR attempts to
fix that oversight.

The `DECAC1` sequence was added in PR #11690, when we disabled `C1`
acceptance by default.

This is a bit of a hack, but I've added a new `AlwaysAcceptC1` mode to
the state machine, which is enabled at startup in the Terminal, and is
never disabled. The parser then just needs to check whether either
`AcceptC1` or `AlwaysAcceptC1` are set.

## Validation Steps Performed

I've manually confirmed the test case in #13968 now works as expected.

Closes #13968

src/cascadia/TerminalCore/Terminal.cpp

@@ -91,7 +91,7 @@ void Terminal::Create(til::size viewportSize, til::CoordType scrollbackLines, Re
     // But if they are being accepted by conhost, there's a chance they may get
     // passed through in some situations, so it's important that our state
     // machine is always prepared to accept them.
-    _stateMachine->SetParserMode(StateMachine::Mode::AcceptC1, true);
+    _stateMachine->SetParserMode(StateMachine::Mode::AlwaysAcceptC1, true);
 }
 
 // Method Description:

src/terminal/parser/stateMachine.cpp

@@ -1716,7 +1716,7 @@ void StateMachine::ProcessCharacter(const wchar_t wch)
         // code points that get translated as C1 controls when that is not their
         // intended use. In order to avoid them triggering unintentional escape
         // sequences, we ignore these characters by default.
-        if (_parserMode.test(Mode::AcceptC1))
+        if (_parserMode.any(Mode::AcceptC1, Mode::AlwaysAcceptC1))
         {
             ProcessCharacter(AsciiChars::ESC);
             ProcessCharacter(_c1To7Bit(wch));

src/terminal/parser/stateMachine.hpp

@@ -50,6 +50,7 @@ namespace Microsoft::Console::VirtualTerminal
         enum class Mode : size_t
         {
             AcceptC1,
+            AlwaysAcceptC1,
             Ansi,
         };