SyncTERM Font Support Detection

* Attempt to detect SyncTERM font support using DSR/CPR
* If a terminal doesn't support DSR/CPR, assume it doesn't support SyncTERM fonts, either.
* Support is also disabled if term simply doesn't ignore the ESC sequence, either

core/client_term.js

@@ -21,6 +21,8 @@ function ClientTerminal(output) {
     //  convert line feeds such as \n -> \r\n
     this.convertLF      = true;
 
+    this.syncTermFontsEnabled = false;
+
     //
     //  Some terminal we handle specially
     //  They can also be found in this.env{}