added SS_LSFT to featured macros

docs/feature_macros.md

@@ -96,6 +96,7 @@ There's also a couple of mod shortcuts you can use:
 * `SS_LCTRL(string)`
 * `SS_LGUI(string)`
 * `SS_LALT(string)`
+* `SS_LSFT(string)`
 
 These press the respective modifier, send the supplied string and then release the modifier.
 They can be used like this:

quantum/quantum.h

@@ -124,6 +124,7 @@ extern uint32_t default_layer_state;
 #define SS_LCTRL(string) SS_DOWN(X_LCTRL) string SS_UP(X_LCTRL)
 #define SS_LGUI(string) SS_DOWN(X_LGUI) string SS_UP(X_LGUI)
 #define SS_LALT(string) SS_DOWN(X_LALT) string SS_UP(X_LALT)
+#define SS_LSFT(string) SS_DOWN(X_LSHIFT) string SS_UP(X_LSHIFT)
 
 #define SEND_STRING(str) send_string_P(PSTR(str))
 extern const bool ascii_to_shift_lut[0x80];