Add g-code quoted strings, improve stream code (#16818)

Marlin/src/gcode/host/M115.cpp

@@ -33,7 +33,9 @@
 #endif
 
 /**
- * M115: Capabilities string
+ * M115: Capabilities string and extended capabilities report
+ *       If a capability is not reported, hosts should assume
+ *       the capability is not present.
  */
 void GcodeSuite::M115() {
 
@@ -41,6 +43,16 @@ void GcodeSuite::M115() {
 
   #if ENABLED(EXTENDED_CAPABILITIES_REPORT)
 
+    // PAREN_COMMENTS
+    #if ENABLED(PAREN_COMMENTS)
+      cap_line(PSTR("PAREN_COMMENTS"), true);
+    #endif
+
+    // QUOTED_STRINGS
+    #if ENABLED(GCODE_QUOTED_STRINGS)
+      cap_line(PSTR("QUOTED_STRINGS"), true);
+    #endif
+
     // SERIAL_XON_XOFF
     cap_line(PSTR("SERIAL_XON_XOFF")
       #if ENABLED(SERIAL_XON_XOFF)
@@ -171,6 +183,5 @@ void GcodeSuite::M115() {
       #endif
     );
 
-
   #endif // EXTENDED_CAPABILITIES_REPORT
 }