Update strikethrough scanDelims to match emphasis

lib/rules_inline/strikethrough.js

@@ -22,7 +22,11 @@ function scanDelims(state, start) {
   lastChar = start > 0 ? state.src.charCodeAt(start - 1) : -1;
 
   while (pos < max && state.src.charCodeAt(pos) === marker) { pos++; }
-  if (pos >= max) { can_open = false; }
+
+  if (pos >= max) {
+    can_open = false;
+  }
+
   count = pos - start;
 
   nextChar = pos < max ? state.src.charCodeAt(pos) : -1;
@@ -31,13 +35,15 @@ function scanDelims(state, start) {
     (isMdAsciiPunct(lastChar) || isPunctChar(String.fromCharCode(lastChar)));
   isNextPunctChar = nextChar >= 0 &&
     (isMdAsciiPunct(nextChar) || isPunctChar(String.fromCharCode(nextChar)));
-  isLastWhiteSpace = lastChar >= 0 && isWhiteSpace(lastChar);
-  isNextWhiteSpace = nextChar >= 0 && isWhiteSpace(nextChar);
+
+  // begin/end of the line counts as a whitespace too
+  isLastWhiteSpace = lastChar < 0 || isWhiteSpace(lastChar);
+  isNextWhiteSpace = nextChar < 0 || isWhiteSpace(nextChar);
 
   if (isNextWhiteSpace) {
     can_open = false;
   } else if (isNextPunctChar) {
-    if (!(isLastWhiteSpace || isLastPunctChar || lastChar === -1)) {
+    if (!(isLastWhiteSpace || isLastPunctChar)) {
       can_open = false;
     }
   }
@@ -45,7 +51,7 @@ function scanDelims(state, start) {
   if (isLastWhiteSpace) {
     can_close = false;
   } else if (isLastPunctChar) {
-    if (!(isNextWhiteSpace || isNextPunctChar || nextChar === -1)) {
+    if (!(isNextWhiteSpace || isNextPunctChar)) {
       can_close = false;
     }
   }

test/fixtures/markdown-it/strikethrough.txt

@@ -95,3 +95,11 @@ test~~</p>
 test
 ~~</p>
 .
+
+From CommonMark test suite, replacing `**` with our marker:
+
+.
+a~~"foo"~~
+.
+<p>a~~“foo”~~</p>
+.