added SHIFT_SPACE_IS_TAB as compile time parameter

for the mobile layout, we want shift+space to produce tab , which needed
to be implemented in the main code. For the desktop layout, we don't want
this though, a config parameter now handles this at compile time.

Ref: https://github.com/jjsullivan5196/wvkbd/pull/103

keyboard.h

@@ -101,6 +101,7 @@ struct kbd {
 	double scale;
 	double preferred_scale, preferred_fractional_scale;
 	bool landscape;
+	bool shift_space_is_tab;
 	uint8_t mods;
 	uint8_t compose;
 	struct key *last_press;