Add ‘eob’ option to fillchars

This option allows configuring what character is shown on the empty
lines at the end of a buffer, previously hardcoded to ‘~’

src/nvim/globals.h

@@ -908,6 +908,7 @@ EXTERN int fill_vert INIT(= 9474);  // │
 EXTERN int fill_fold INIT(= 183);   // ·
 EXTERN int fill_diff INIT(= '-');
 EXTERN int fill_msgsep INIT(= ' ');
+EXTERN int fill_eob INIT(= '~');
 
 /* Whether 'keymodel' contains "stopsel" and "startsel". */
 EXTERN int km_stopsel INIT(= FALSE);