patch 8.0.1038: strike-through text not supported

Problem:    Strike-through text not supported.
Solution:   Add support for the "strikethrough" attribute. (Christian
            Brabandt, Ken Takata)
diff --git a/src/term.h b/src/term.h
index 0fedfea..b95b4f6 100644
--- a/src/term.h
+++ b/src/term.h
@@ -55,6 +55,8 @@
     KS_US,	/* underscore (underline) mode */
     KS_UCE,	/* exit undercurl mode */
     KS_UCS,	/* undercurl mode */
+    KS_STE,	/* exit strikethrough mode */
+    KS_STS,	/* strikethrough mode */
     KS_MS,	/* save to move cur in reverse mode */
     KS_CM,	/* cursor motion */
     KS_SR,	/* scroll reverse (backward) */
@@ -149,6 +151,8 @@
 #define T_US	(TERM_STR(KS_US))	/* underscore (underline) mode */
 #define T_UCE	(TERM_STR(KS_UCE))	/* exit undercurl mode */
 #define T_UCS	(TERM_STR(KS_UCS))	/* undercurl mode */
+#define T_STE	(TERM_STR(KS_STE))	/* exit strikethrough mode */
+#define T_STS	(TERM_STR(KS_STS))	/* strikethrough mode */
 #define T_MS	(TERM_STR(KS_MS))	/* save to move cur in reverse mode */
 #define T_CM	(TERM_STR(KS_CM))	/* cursor motion */
 #define T_SR	(TERM_STR(KS_SR))	/* scroll reverse (backward) */